drm/msm: Drop struct_mutex in madvise path

The obj->lock is sufficient for what we need.

This *does* have the implication that userspace can try to shoot
themselves in the foot by racing madvise(DONTNEED) with submit.  But
the result will be about the same if they did madvise(DONTNEED) before
the submit ioctl, ie. they might not get want they want if they race
with shrinker.  But iova fault handling is robust enough, and userspace
is only shooting it's own foot.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>