msm: mdss: Flush Interface timing registers only once

MDSS interface timing engine registers need to be flushed
only once during the first commit to the respective panel.

Change-Id: I7a619f2eeeb57baf5429346a11eb41eedac252f3
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>