drm/i915/skl: Enable pipe gamma for sprite planes.

Since SKL has universal planes, we should configure the sprite planes
and the primary plane the same.  For the primary plane we do enable
the pipe gamma on the plane so do the same for the non-primary planes.

Without this, the pipe CRC values will be different for something
displayed on the primary plane and something displayed on a sprite
plane when the ARGB8888 format is used.

Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>