drm: omapdrm: Use a spinlock to protect the CRTC pending flag

The CRTC pending flag will need to be accessed atomically in the vblank
interrupt handler, memory barriers won't be enough to protect it. Use a
spinlock instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>