drm/ast: do not attempt to acquire a reservation while in an interrupt handler

Mutexes should not be acquired in interrupt context. While the trylock
fastpath is arguably safe on all implementations, the slowpath
unlock path definitely isn't.

Signed-off-by: Maarten Lankhorst <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/ast/ast_fb.c

···
 	struct ast_bo *bo;
 	int src_offset, dst_offset;
 	int bpp = (afbdev->afb.base.bits_per_pixel + 7)/8;
-	int ret;
+	int ret = -EBUSY;
 	bool unmap = false;
 	bool store_for_later = false;
 	int x2, y2;
···
 	 * then the BO is being moved and we should
 	 * store up the damage until later.
 	 */
-	ret = ast_bo_reserve(bo, true);
+	if (!in_interrupt())
+		ret = ast_bo_reserve(bo, true);
 	if (ret) {
 		if (ret != -EBUSY)
 			return;