drm/msm: Make msm_gem_free_object() static

Misc small cleanup I noticed.  Not called from another object file since
commit 3c9edd9c

 ("drm/msm: Introduce GEM object funcs")

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489362/
Link: https://lore.kernel.org/r/20220613204910.2651747-1-robdclark@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>