drm/msm: Switch idr_lock to spinlock

Needed to idr_preload() which returns with preemption disabled.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/527846/
Link: https://lore.kernel.org/r/20230320144356.803762-11-robdclark@gmail.com