msm: mdss: Avoid adaptive variable refresh during dynaminc FPS

Both dynamic FPS (DFPS) and adaptive variable refresh (AVR) depend
and modify the programmable fetch. During DFPS update period, which
is between 2 VSYNC, it is necessary to disable the AVR during this
period, otherwise it will cause the timing engine hangs.

CRs-Fixed: 1073650
Change-Id: I2836ef5b7f37f627a32a0b4332dc299a60012704
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>