drm/msm: Init mm_list before accessing it for use_vram path

Fix NULL pointer dereference caused by update_inactive()
trying to list_del() an uninitialized mm_list who's
prev/next pointers are NULL.

Fixes: 64fcbde7

 ("drm/msm: Track potentially evictable objects")
Signed-off-by: Alexey Minnekhanov <alexeymin@postmarketos.org>
Link: https://lore.kernel.org/r/20210518102624.1193955-1-alexeymin@postmarketos.org
Signed-off-by: Rob Clark <robdclark@chromium.org>