drm/amdgpu: Use amdgpu by default for CIK APUs too

drm/amdgpu: Use amdgpu by default for CIK APUs too

CIK APUs are: Kaveri, Kabini and Mullins from 2013~2015,
which all have a second generation GCN based integrated GPU.

The amdgpu driver has been working well on CIK APUs for years.
Features which were previously missing have been added recently,
specifically DC support for analog connectors and DP bridge
encoders. Now amdgpu is at feature parity with the old radeon
driver on CIK APUs.

Enabling the amdgpu driver by default for CIK APUs 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

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