msm_fb: display: Fix flickering when BLT is enabled/disabled

The transition of BLT enable and disable start at the beginning
of vsync. This ensures that the transition is completed within the
same vsync period. We Wait for DMA-P done after turning off the
timing generator and do not prefill the frame.

Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>