drm/i915: Simplify shrinker locking

By using the same structure for both interruptible and
uninterruptible locking in shrinker code, combined with the
information that mm.interruptible is only being written to, the
code can be greatly simplified.

Also removing the i915_gem_ prefix from the locking functions so
that nobody in their wildest dreams considers exporting them.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1491562175-27680-1-git-send-email-joonas.lahtinen@linux.intel.com