drm/i915/fbc: Nuke fbc_supported()

fbc_supported() is just a pointless wrapper for HAS_FBC(). Get
rid of it. In places where we're operating on a specific plane
we can replace this with a plane->has_fbc check to avoid
doing anything for crtcs that don't even support fbc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213133453.22152-4-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>