drm/amdgpu: Use DC by default on CIK APUs

drm/amdgpu: Use DC by default on CIK APUs

Now that DC supports external DP bridge encoders,
it has reached feature parity with the legacy non-DC display
driver on CIK APUs: Kaveri, Kabini, Mullins.

Use the DC display driver by default on SI APUs, unless it is
explicitly disabled using the amdgpu.dc=0 module parameter.

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

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