drm/amd/display: When blanking during init loop to find OPP index

[Description]
For pipe harvesting cases we cannot rely on array index
to get the correct OPP instance, we must loop through
each instance to find the correct one.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>