drm/msm: Drop queued submits on lastclose()

drm/msm: Drop queued submits on lastclose()

If we haven't written the submit into the ringbuffer yet, then drop it.
The submit still retires through the normal path, to preserve fence
signalling order, but we can skip the IB's to userspace cmdstream.

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/661489/