drm/lima: handle shared irq case for lima_pp_bcast_irq_handler

On Hikey board all lima ip blocks are shared with one irq.
This patch avoids a NULL ptr deref crash on this platform
on startup. Tested with Weston and kmscube.
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1555662781-22570-7-git-send-email-peter.griffin@linaro.org

drivers/gpu/drm/lima/lima_pp.c

@@ -64,7 +64,13 @@ static irqreturn_t lima_pp_bcast_irq_handler(int irq, void *data)
 	struct lima_ip *pp_bcast = data;
 	struct lima_device *dev = pp_bcast->dev;
 	struct lima_sched_pipe *pipe = dev->pipe + lima_pipe_pp;
-	struct drm_lima_m450_pp_frame *frame = pipe->current_task->frame;
+	struct drm_lima_m450_pp_frame *frame;
+
+	/* for shared irq case */
+	if (!pipe->current_task)
+		return IRQ_NONE;
+
+	frame = pipe->current_task->frame;
 
 	for (i = 0; i < frame->num_pp; i++) {
 		struct lima_ip *ip = pipe->processor[i];