drm/i915: Generalize the async flip capability check

drm/i915: Generalize the async flip capability check

Only assign the plane->async_flip() vfunc when the plane supports
async flips. For now we keep this artificially limited to the primary
plane since thats the only thing the legacy page flip uapi can target
and there is no async flip support in the atomic uapi yet.

Cc: Karthik B S <karthik.b.s@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111163711.12913-5-ville.syrjala@linux.intel.com
Reviewed-by: Karthik B S <karthik.b.s@intel.com>