drm/amdgpu: default to true in amdgpu_device_asic_has_dc_support

We are not going to support any new chips with the old
non-DC code so make it the default.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>