drm/msm/sde: 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: I5b9187c7961c6ecc7dde88aba6ef705af7c4e15c
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>