drm/msm/gem: Add missing rcu_dereference()

Fixes a sparse "different address spaces" error.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406280050.syeEwLTE-lkp@intel.com/
Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/601612/