drm/amd/display: Change HDMI judgement condition.

[Why & How]
Use dc_is_hdmi_signal to determine signal type.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: JinZe.Xu <JinZe.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>