drm/amd/display: Read DPCD to obtain eDP capability information.

[Why & How]
Extend to read eDP general capability 2 in detect_edp_sink_caps().

Reviewed-by: Robin Chen <robin.chen@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Allen Li <wei-guang.li@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>