drm/amd/display: Read LTTPR ALPM caps during link cap retrieval

[Why]
The latest DP spec requires the DP TX to read DPCD F0000h through F0009h
when detecting LTTPR capabilities for the first time.

[How]
Update LTTPR cap retrieval to read up to F0009h (two more bytes than the
previous F0007h), and store the LTTPR ALPM capabilities.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>