drm/msm/gpu: make ringbuffer readonly

[ Upstream commit 352c83fb39cae3eff95a8e1ed23006291abb6196 ]

The GPU has no business writing into the ringbuffer, let's make it
readonly to the GPU.

Fixes: 7198e6b0 ("drm/msm: add a3xx gpu support")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>