drm/i915: Eliminate the RMW sprite colorkey management

Store the colorkey in intel_plane and kill off all the RMW stuff
handling it.

This is just an intermediate step and eventually the colorkey needs to
be converted into some properties.

v2: Actually update the hardware state in the set_colorkey ioctl (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>