drm/msm: Drop priv->lastctx

drm/msm: Drop priv->lastctx

cur_ctx_seqno already does the same thing, but handles the edge cases
where a refcnt'd context can live after lastclose.  So let's not have
two ways to do the same thing.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Akhil P Oommen <akhilpo@codeaurora.org>
Link: https://lore.kernel.org/r/20211109181117.591148-3-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>