msm: mdss: Fix AVR flush for split control path

For split control display configuration, it is necessary to flush the
timing engine for both control path. The change is for fixing the
flush bit on the second control path.

CRs-Fixed: 2010398
Change-Id: Ie12e6c6718b2061908e13c9ccd97d375c594fa00
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>