drm/amd/display: read invalid ddc pin status cause engine busy

[Why]
There is no DDC_6 pin on new asic cause the mapping table is
incorrect. When app try to access DDC_VGA port, driver read
an invalid ddc pin status and report engine busy.

[How]
Add dummy DDC_6 pin to align gpio structure.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Paul Hsieh <Paul.Hsieh@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>