drm: Add support for tracking drawable information to core

Actually make the existing ioctls for adding and removing drawables do
something useful, and add another ioctl for the X server to update drawable
information. The only kind of drawable information tracked so far is cliprects.

Only reallocate cliprect memory if the number of cliprects changes.
Also improve diagnostic output.

hook up drm ioctl update draw
export drm_get_drawable_info symbol

Signed-off-by: Dave Airlie <airlied@linux.ie>