drm/amdgpu: Use DC by default on SI dGPUs

drm/amdgpu: Use DC by default on SI dGPUs

Now that DC supports analog connectors, it has reached feature
parity with the legacy non-DC display driver on SI dGPUs.
Use the DC display driver by default on SI dGPUs, unless it is
explicitly disabled using the amdgpu.dc=0 module parameter.

DC brings proper support for DP/HDMI audio, DP MST,
10-bit colors, some HDR features, atomic modesetting, etc.

Also clarify the comment about what is missing to have full
DC support for CIK APUs.

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