drm/amd/display: Determine synchronization edge based on master's vsync

[Why&How]
The driver always wants to synchronize streams
to the first edge of master's vsync pulse.
In order to determine that we can read timing
flags that are used to program vsync.

Master stream's vsync polarity - Multi Display Stream Synchronization edge:

        Negative               -              Falling Edge

        Positive               -              Rising Edge

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>