drm/amd/display: get socBB from VBIOS

[why]
Some SOC BB paramters may vary per SKU, and it does
not make sense for driver to hardcode these values

[how]
Parse the values from VBIOS if available, and use
them if valid

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>