drm: Drop all the stub gamma_get, gamma_set, load_lut functions from drivers

Many of the drivers didn't implement palette/gamma handling, but were forced
to provide stubs for the hooks to avoid drm_fb_helper from oopsing. Now that
the hooks are optional, we can eliminate all the stubs.

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