drm/amd/display: Implement voltage limitation stub

add new function to get the voltage at the end of
dcn_validate_bandwidth, to check against the
highest voltage we allow.

Created a stub to allow for optimizations

Signed-off-by: Joseph Gravenor <joseph.gravenor@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Sun peng Li <Sunpeng.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>