drm/msm/rd: Fix FIFO-full deadlock

If the previous thing cat'ing $debugfs/rd left the FIFO full, then
subsequent open could deadlock in rd_write() (because open is blocked,
not giving a chance for read() to consume any data in the FIFO). Also
it is generally a good idea to clear out old data from the FIFO.

Signed-off-by: Rob Clark <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/496706/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/msm_rd.c

···
 	file->private_data = rd;
 	rd->open = true;
 
+	/* Reset fifo to clear any previously unread data: */
+	rd->fifo.head = rd->fifo.tail = 0;
+
 	/* the parsing tools need to know gpu-id to know which
 	 * register database to load.
 	 *