drm/amd/display: Use the largest vready_offset in pipe group

[WHY]
Corruption can occur in LB if vready_offset is not large enough.
DML calculates vready_offset for each pipe, but we currently select the
top pipe's vready_offset, which is not necessarily enough for all pipes
in the group.

[HOW]
Wherever program_global_sync is currently called, iterate through the
entire pipe group and find the highest vready_offset.

Reviewed-by: Dillon Varone <Dillon.Varone@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>