drm/msm/rd: Fix FIFO-full deadlock

stable inclusion
from stable-v5.10.144
commit cff2b3a50c69c0c72593f87190c88d0235df6503
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6D0V7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cff2b3a50c69c0c72593f87190c88d0235df6503



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[ Upstream commit 174974d8 ]

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 <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496706/
Link: https://lore.kernel.org/r/20220807160901.2353471-2-robdclark@gmail.com


Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>