drm/i915/fbc: Don't clear busy_bits for origin==GTT

drm/i915/fbc: Don't clear busy_bits for origin==GTT

The hardware host tracking won't nuke the entire cfb (unless the
entire fb is written through the gtt) so don't clear the busy_bits
for gtt tracking.

Not that it really matters anymore since we've lost ORIGIN_GTT usage
everywhere.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429101034.8208-7-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>