drm/i915: Intel-specific primary plane handling (v4)

Intel hardware allows the primary plane to be disabled independently of
the CRTC.  Provide custom primary plane handling to allow this.

v4:
 - Don't add a primary_plane field to intel_crtc; that was left over
   from a much earlier iteration of this patch series, but is no longer
   needed/used now that the DRM core primary plane support has been
   merged.
v3:
 - Provide gen-specific primary plane format lists (suggested by Daniel
   Vetter).
 - If the primary plane is already enabled, go ahead and just call the
   primary plane helper to do the update (suggested by Daniel Vetter).
 - Don't try to disable the primary plane on destruction; the DRM layer
   should have already taken care of this for us.
v2:
 - Unpin fb properly on primary plane disable
 - Provide an Intel-specific set of primary plane formats
 - Additional sanity checks on setplane (in line with the checks
   currently being done by the DRM core primary plane helper)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>