drm/i915: Reserve ioctl numbers for set/get_caching

I'm planing to merge this next week for 3.7, but I'd like to avoid
stupid conflicts with the exsting userspace when merging the new
reg_read ioctl (which doesn't have userspace yet, but this caching
interface has).

Header extracted from Chris Wilson's patch, but fix up the copy&pasted
comment in the interface struct.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>