drm: Rip out the racy, unused vblank signal code.

Schedule a vblank signal, kill the process, and we'll go walking over freed
memory.  Given that no open-source userland exists using this, nor have I
ever heard of a consumer, just let this code die.

Signed-off-by: Eric Anholt <eric@anholt.net>
Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>