drm/msm/a5xx: Skip hardware preemption init if no preemption

In the case where preemption is not enabled, this patch simply skips
preemption related initialization in hardware init sequence.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>