drm/i915: Use fb format modifiers in skylake_update_primary_plane

Just a little demo really. We probably need to introduce skl specific
functions for a lot of the format validation stuff, or at least
helpers. Specifically I think intel_framebuffer_init and
intel_fb_align_height must be adjusted to have an i915_ and a skl_
variant. And only shared code should be converted to fb modifiers,
platform code (like the plane config readout can keep on using old
tiling_mode defines to avoid some churn).

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>