drm/msm: unlock on error in msm_gem_get_iova()

We recently added locking to this function but there was a direct return
that was overlooked where we need to unlock.

Fixes: 0e08270a1f01 ("drm/msm: Separate locking of buffer resources from struct_mutex")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>