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,
amdgpu is at feature parity with the old radeon driver
on CIK dGPUs.

Enabling the amdgpu driver by default for CIK dGPUs has the
following benefits:

- More stable OpenGL support through RadeonSI
- Vulkan support through RADV
- Improved performance
- Better display features through DC

Users who want to keep using the old driver can do so using:
amdgpu.cik_support=0 radeon.cik_support=1

v2:
- Update documentation in Kconfig file
v3:
- Rebase documentation updates (Alex)

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