msm: mdss: add support for adaptive variable refresh feature

Adaptive variable refresh(AVR) allows timing modification of a current
frame by extending the vertical front porch values. This helps
shifting the vsync to slowest vsync interval supported by panel when
GPU rendering ready event is delayed. Define an interface for AVR to
enable client to control AVR settings. Validate client input for AVR
and configure the corresponding AVR registers.

CRs-Fixed: 1043819
Change-Id: I28f786bf0bdea4ecaa17270974ba6d7ffe469828
Signed-off-by: Ramkumar Radhakrishnan <ramkumar@codeaurora.org>