msm_fb: display: Set timing generator after register flush

Timing generator was getting started before the register flush.
Stale values were sometimes causing IOMMU page faults on bootup.

Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>
Change-Id: Ia671c3603e383af94197a23a8e5e5f32274a9978