drm/msm: use trylock for debugfs

This resolves a potential deadlock vs msm_gem_vm_close().  Otherwise for
_NO_SHARE buffers msm_gem_describe() could be trying to acquire the
shared vm resv, while already holding priv->obj_lock.  But _vm_close()
might drop the last reference to a GEM obj while already holding the vm
resv, and msm_gem_free_object() needs to grab priv->obj_lock, a locking
inversion.

OTOH this is only for debugfs and it isn't critical if we undercount by
skipping a locked obj.  So just use trylock() and move along if we can't
get the lock.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661525/

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