disp: msm: sde: move qsync validation to encoder check

In connector atomic check, seamless vrr cannot be
determined since mode doesn't get populated by that time.
This change moves qsync validation from connector to encoder
atomic check to verify and handle modeset concurrencies.

Change-Id: Ibd3a682b01e08f99d0a0eb1ca2ba51a7c8fe5e76
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>