drm/msm: Document and rename preempt_lock

[ 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>