msm: mdss: Use single flush method for 8084 split display

Split display use-cases have two independent control paths. MDSS before
8084 requires both control paths to program their own flush registers
which can lead to race conditions. Starting 8084, MDSS HW can achieve
same flush but with only single register write and eliminating race
conditions.

Change-Id: I62e5d61decb6f20cbd9bec542e035471b84b2c0a
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>