staging: mrst: Add overlay color keying and constant alpha

staging: mrst: Add overlay color keying and constant alpha

Both source and destination color keying can be enabled for the
overlays.

Also the overlay constant alpha blending factor can be changed.
Note that the overlay constant alpha isn't all that useful, though.
What you usually want is to control graphics alpha. That will need
to be handled through the CRTC somehow (a new ioctl possibly) until
such time when the scanout functionality can be ripped out of the
CRTC object. When that happens the graphics plane would be just another
drm plane.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>