drm/amd/display: disable ext aux support for vega

[Why]
Earlier changes to support configurable aux timeout
caused dc init failure on vega due to missing reg defs.
Needs to be disabled until implemented for vega.

[How]
Set extended aux timeout cap for vega to false.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-By: abdoulaye berthe <abdoulaye.berthe@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>