drm/i915: Enable pipe gamma for sprites

We send the primary and cursor plane data through the gamma unit.
In order to get matching output from sprites, also send the sprite
data through the gamma unit.

In the future we should add some properties to control this
explicitly, and also add properties for the per-sprite gamma ramps
what have you, but for now this seems like a reasonable thing to do.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>