drm/msm: Fix premature purging of BO

In the case where we have a back-to-back submission that shares the same
BO, this BO will be prematurely moved to inactive_list while retiring the
first submit. But it will be still part of the second submit which is
being processed by the GPU. Now, if the shrinker happens to be triggered at
this point, it will result in a premature purging of this BO.

To fix this, we need to refcount BO while doing submit and retire. Then,
it should be moved to inactive list when this refcount becomes 0.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>