drm/amd/display: Use helper function to check for HDMI signal

[How]
Use dc_is_hdmi_signal to determine signal type.

Signed-off-by: JinZe.Xu <JinZe.Xu@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>