drm/msm: Drop priv->lastctx

drm/msm: Drop priv->lastctx

[ Upstream commit 1d054c9b8457b56a651109fac21f56f46ccd46b2 ]

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>
Stable-dep-of: a30f9f65b5ac ("drm/msm/a5xx: workaround early ring-buffer emptiness check")
Signed-off-by: Sasha Levin <sashal@kernel.org>