drm/amd/display: initialize get_max_link_cap

drm/amd/display: initialize get_max_link_cap

[why]
usb3->usb2 switch system hang.
driver needs to limit the max sink cap based on DP4 mode.

[how]
based on s_dpalt check and DP4 check:
limit the USB-C DPALT DP maximum supported lane count.

Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>