drm/i915: Grab execlist spinlock to avoid post-reset concurrency issues.

Grab execlist lock when cleaning up execlist queues after GPU reset to avoid
concurrency problems between the context event interrupt handler and the reset
path immediately following a GPU reset.

* v2 (Chris Wilson):
Do execlist check and use simpler form of spinlock functions.

Signed-off-by: Tomas Elf <tomas.elf@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>