drm/i915: Rip out GET_SPRITE_COLORKEY ioctl

It's completely unused and Tommi noticed that the #define is borked
since forever. I've done a git search in userspace and only found
broken definitions and no users anywhere.

Cc: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>