drm/i915: Clarify mmio_flip_lock locking

The ->queue_flip callback is always called from process context, so
plain _irq spinlock variants are enough.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge