drm/amd/display: Rename is_hdmi to is_hdmi_tmds type

HDMI has TMDS and FRL signal types. Be specific about what is used.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>