drm/msm: Remove dangling submitqueue references

Currently it doesn't matter, since we free the ctx immediately.  But
when we start refcnt'ing the ctx, we don't want old dangling list
entries to hang around.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>