drm/i915: Don't bother with the BKL for GEM ioctls.

We probably don't need it for most of the other driver ioctls as well,
but we explicitly did locking when doing the GEM pieces.  On CPU-bound
graphics tasks, the BKL was showing up as 1-2% of CPU time.

Signed-off-by: Eric Anholt <eric@anholt.net>