msm: mdss: support VRR when destination split is enabled

When destination split is enabled, we have a single control path
and two interface timing engines. Add changes to support variable
refresh rate (VRR) using vertical porch method for these cases.

Change-Id: Iad78c2282e7620454eddf839ed94618731b9621b
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>