drm/i915: Fix unclaimed register access due to delayed VGA memory disable

drm/i915: Fix unclaimed register access due to delayed VGA memory disable

VGA registers live inside the power well on HSW, so in order to write
the VGA MSR register we need the power well to be on.

We really must write to the register to properly clear the
VGA_MSR_MEM_EN enable bit, even if all VGA registers get zeroed when
the power well is down. It seems that the implicit zeroing done by
the power well is not enough to propagate the VGA_MSR_MEM_EN bit to
whomever is actually responsible for the memory decode ranges.

If we leave VGA memory decode enabled, and then turn off the power well,
all VGA memory reads will return zeroes. But if we first disable VGA
memory deocde and then turn off the power well, VGA memory reads
return all ones, indicating that the access wasn't claimed by anyone.
For the vga arbiter to function correctly the IGD must not claim the
VGA memory accesses.

Previously we were doing the VGA_MSR register access while the power well
was excplicitly powered up during driver init. But ever since

 commit 6e1b4fdad5157bb9e88777d525704aba24389bee
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Thu Sep 5 20:40:52 2013 +0300

    drm/i915: Delay disabling of VGA memory until vgacon->fbcon handoff is done

we delay the VGA memory disable until fbcon has initialized, and so
there's a possibility that the power well got turned off during the
fbcon modeset. Also vgacon_save_screen() will need the power well to be
on to be able to read the VGA memory.

So immediately after enabling the power well during init grab a refence
for VGA purposes, and after all the VGA handling is done, release it.

v2: Add intel_display_power_put() for the num_pipes==0 case

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix up the patch wiggle screw-up that I've done and which
Paulo catched. Also polish spelling in the patch headline.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>