drm/amd/display: Fix soft hang issue when some DPCD data invalid

[Why]
AUX transaction returns success, but data has invalid lane count and rate
which when passed to VBIOS command table causes it to soft hang

[How]
Do some sanity checking and fail if the DPCD caps are invalid.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>