msm: mdss: Enable dynamic FPS support for msmcobalt

Adding support for dynamic FPS adjustment in mdss 3.x. Adjustment
is based on front/back porch changes.

CRs-Fixed: 1047142
Change-Id: Ie4ab889a7d64ba8e33675ee22bf7b808d9ba0a3f
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>