drm/msm: Add a context pointer to the submitqueue

Each submitqueue is attached to a context. Add a pointer to the
context to the submitqueue at create time and refcount it so
that it stays around through the life of the queue.

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