drm/msm/dsi-staging: take struct mutex before gem free

Take the device struct mutex before freeing a gem buffer using
msm_gem_free_object, which requires that mutex to be held.

Change-Id: Idb8729c4734ba96e4b5f7e912ef85b11648bb6c9
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>