msm_fb: display: Flush MDP registers before staging up any pipe

Add flush calls to MDP's pipe and mixer registers before staging
up any pipe to a mixer.

Change-Id: I36f5aa04634bae43a38e57cd97bd2aafe9631eda
Signed-off-by: Ravishangar Kalyanam <rkalya@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>