drm/i915: Cleanup i915_param()

drm/i915: Cleanup i915_param()

Rather than having a separate case for each value where we just return
a hardcoded value = 1, we lump them all together and rely on the awesome
case-fallthrough feature of C.

Fix all feature macros to pass dev_priv instead of dev while at it,
and use INTEL_GEN() instead of INTEL_INFO()->gen.

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160902104617.29089-1-david.weinehall@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>