drm/amd/display: Add timing validation against dongle cap

drm/amd/display: Add timing validation against dongle cap

For DP active dongles, the dpcd dongle caps are read but not
used to validate mode timing. This addresses this.

In particular, this change fixes light up on the HDMI 4k TV
connected through DP active dongle. Since the 4k TV defaults
to YCbCr420, which the dongle don't support.

This change does not address MST cases, a more generalized
approach must be taken for that.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>