drm/amdgpu: Use amdgpu by default on CIK dedicated GPUs

drm/amdgpu: Use amdgpu by default on CIK dedicated GPUs

The amdgpu driver has been working well on CIK dGPUs for years.
Now that the DC analog connector support landed, these GPUs
are at feature parity with the old radeon driver.

Additionally, amdgpu yields extra performance, supports Vulkan
and provides more display features through DC as well as more
robust power management.

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