drm/amd/display: Guard ASSR with internal display flag

[Why]
ASSR enabling only considers capability declared in DPCD.
We also need to check whether the connector is internal.

[How]
ASSR enabling need to check both DPCD capability and internal display
flag.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>