drm/i915: Don't assume primary & cursor are always on for wm calculation (v4)

drm/i915: Don't assume primary & cursor are always on for wm calculation (v4)

Current ILK-style watermark code assumes the primary plane and cursor
plane are always enabled.  This assumption, along with the combination
of two independent commits that got merged at the same time, results in
a NULL dereference.  The offending commits are:

        commit fd2d61341bf39d1054256c07d6eddd624ebc4241
        Author: Matt Roper <matthew.d.roper@intel.com>
        Date:   Fri Feb 27 10:12:01 2015 -0800

            drm/i915: Use plane->state->fb in watermark code (v2)

and

        commit 0fda65680e92545caea5be7805a7f0a617fb6c20
        Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
        Date:   Fri Feb 27 15:12:35 2015 +0000

            drm/i915/skl: Update watermarks for Y tiling

The first commit causes us to use the FB from plane->state->fb rather
than the legacy plane->fb, which is updated a bit later in the process.

The second commit includes a change that now triggers watermark
reprogramming on primary plane enable/disable where we didn't have one
before (which wasn't really correct, but we had been getting lucky
because we always calculated as if the primary plane was on).

Together, these two commits cause the watermark calculation to
(properly) see plane->state->fb = NULL when we're in the process of
disabling the primary plane.  However the existing watermark code
assumes there's always a primary fb and tries to dereference it to find
out pixel format / bpp information.

The fix is to make ILK-style watermark calculation actually check the
true status of primary & cursor planes and adjust our watermark logic
accordingly.

v2: Update unchecked uses of state->fb for other platforms (pnv, skl,
    etc.).  Note that this is just a temporary fix.  Ultimately the
    useful information is going to be computed at check time and stored
    right in the state structures so that we don't have to figure this
    all out while we're supposed to be programming the watermarks.
    (caught by Tvrtko)

v3: Fix a couple copy/paste mistakes in SKL code. (Tvrtko)

v4: Only add FB checks for ILK/SKL codepaths.  Older platforms still use
    intel_crtc_active() and will shortcircuit out of watermark
    calculations before ever trying to dereference the primary plane's
    framebuffer.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Michael Leuchtenburg <michael@slashhome.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89388
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>