drm/i915: Tighten the exposure ARGB/ABGR 8888 formats

ARGB8888 is used for cursors on all platforms so we need to allow it
everywhere.

ABGR8888 is currently only honoured:
  - on VLV/CHV in sprite planes
  - on SKL+ for primary and sprite planes
so only allow it for those platforms.

Note that we only support ARGB8888/ABGR8888 on the primary plane for
SKL/BXT because we have in line of sight the pipe bottom color on those
platforms and because the primary plane programming on VLV/CHV doesn't
anything different for those formats today.

v2: Fix the logic to forbid the creation ABGR2101010 fbs (Ville)
v3: Still allow the creation of ARGB8888 fbs now that cursor planes use
    real fb objects (found by PRTS).

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>