drm/amdgpu: log when amdgpu.dc=1 but ASIC is unsupported

This makes it easier to figure out whether the kernel parameter has been
taken into account.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>