drm/i915: more struct_mutex locking

When auditing the locking in i915_gem.c (for a prospective change which I
then abandoned), I noticed two places where struct_mutex is not held
across GEM object manipulations that would usually require it.  Since one
is in initial setup and the other in driver unload, I'm guessing the mutex
is not required for either; but post a patch in case it is.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Keith Packard <keithp@keithp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Keith Packard <keithp@keithp.com>