mdss: display: Add support for dynamic FPS

Add sysfs entry for dynamic FPS update mode. Add support to
change FPS in the following scenarios:
1. Store the FPS and change the clocks only when Suspend/resume
   happens. This is the default configuration.
2. Change the FPS immediately by updating the clocks. Video
   mode engine is disabled and then enabled when clock rate
   is updated.

CRs-Fixed: 488453
Change-Id: I1722d54d9fbfdfbd7d22e02d8776a0f60dc86141
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>