drm/amd/display: fix dp kvm can't light up

[why]
The DP KVM failed to light up, since the lttpr_mode is not reset to
default value when failed to read LTTPR capabilities, and the
variable max_link_rate retains a initial value zero, this cause variable
link_rate be assigned to an error value zero, consquently pixel_clock
get wrong value zero, and kvm can't light up.

[how]
Reset lttpr_mode to default value when failed to read LTTPR
capabilities, so that the link_rate fallbacks to the minimum rate that
supported by link encoder and sink.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>