drm/amd/display: Use dc_is_hdmi_signal() instead of ENUM

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>