drm/i915/fbc: s/gen9 && !glk/gen9_bc || bxt/

Replace the 'gen9 && !glk' with the slightly more obvious
'gen9_bc || bxt'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-10-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>