drm/amd/display: update conditions to do dfp cap ext validation

drm/amd/display: update conditions to do dfp cap ext validation

[why]
According to DP specs dfp cap ext validation is only for branch
device withou 128b/132b channel coding support and
downstream of the branch device doesn't have compression.
Therefore we are adding conditions to only do dfp cap
extension validation for branch devcie supporting 8b/10b
channel coding only and it has no DSC passthrough capability.

Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>