drm/msm: Document and rename preempt_lock

stable inclusion
from stable-v5.10.175
commit 8dde1d9d6f065ea4c03c1d77182ba9e122d516d3
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8711T

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8dde1d9d6f065ea4c03c1d77182ba9e122d516d3



--------------------------------

[ Upstream commit 77c40603 ]

Before adding another lock, give ring->lock a more descriptive name.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Stable-dep-of: b4fb748f ("drm/msm/a5xx: fix the emptyness check in the preempt code")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>