drm/amd/display: Get num_chans from VBIOS table

Get the values from VBIOS table

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>