drm/amd/display: Read DPCD link caps up to and including DP_ADAPTER_CAP

DP 1.4 compliance requires 16 bytes to be read when reading link caps,
i.e. it requires DP_ADAPTER_CAP to be included. Included it for all DP
versions because reading more than required won't fail.

Signed-off-by: Nikola Cornij <nikola.cornij@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>