drm/msm: Drop context arg to gpu->submit()

Now that we can get the ctx from the submitqueue, the extra arg is
redundant.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
[split out of previous patch to reduce churny noise]
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>