drm/amd/display: Determine early if a link has supported encoders (v2)

drm/amd/display: Determine early if a link has supported encoders (v2)

Avoid initializing DDC, HPD, etc. when we know that the link is
not going to be constructed because it has no supported encoders.

This is mainly useful for old GPUs which may have encoders such
as TRAVIS and NUTMEG that are not yet supported by DC.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>