drm/msm: Unblock writer if reader closes file

[ Upstream commit 99c66bc051e7407fe0bf0607b142ec0be1a1d1dd ]

Prevents deadlock when fifo is full and reader closes file.

Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>