drm/msm: Split out idr_lock

Otherwise if we hit reclaim pinning objects in the submit path, we'll be
blocking retire_worker trying to free a submit.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496116/
Link: https://lore.kernel.org/r/20220802155152.1727594-4-robdclark@gmail.com