drm/amd/display: Clean up global sync param retrieval

[Why]
This change replaces older looping code in favor of these functions.

[How]
There are built in functions for extracting global sync params
during mode validation now.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>