drm/i915: Expose alpha formats on VLV/CHV primary planes

Currently we expose VLV/CHV alpha blending only on the sprite
planes, but the primary planes can do it as well. Let's flip
it on.

v2: Rebase due to fp16 landing

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191031165652.10868-3-ville.syrjala@linux.intel.com