drm/msm: Disable preemption on all 5xx targets

Temporarily disable preemption on a5xx targets pending some improvements
to protect the RPTR shadow from being corrupted.

Cc: stable@vger.kernel.org
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>