drm/i915/skl: don't fail colorkey + scaler request

There is a mplayer video failure reported with xv.
This is because there is a request to do both plane scaling
and colorkey. Because skl hw doesn't support plane scaling
and colorkey at the same time, request is failed which is expected
behavior.

To make xv operate, this patch allows colorkey continue to work
without using scaler. Then behavior would be similar to platforms
without plane scaler support.

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90449


[danvet: change can_scale to bool as requested by Ville.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>