drm/msm/gpu: Add dev_to_gpu() helper

In a later patch, the drvdata will not directly be 'struct msm_gpu *',
so add a helper to reduce the churn.

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