drm/i915: fix definition of the DRM_IOCTL_I915_GET_SPRITE_COLORKEY ioctl

Fix definition of the DRM_IOCTL_I915_GET_SPRITE_COLORKEY ioctl, so that it
is different from the DRM_IOCTL_I915_SET_SPRITE_COLORKEY ioctl.

Note that this is just for accuracy, the ioctl implementation itself is totally
unused and already ripped out.

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
[danvet: Add note that this is a dead ioctl.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>