drm/i915: split fb scalable checks into g4x and skl versions

This just cleans these up a bit.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c91d924e93965515d2017dbf3c89c245ff6d52ea.1612536383.git.jani.nikula@intel.com