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 <[e-mail]>
Signed-off-by: Timur Kristóf <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/Kconfig

···
 	  Choose this option if you want to enable support for CIK (Sea
 	  Islands) asics.
 
-	  CIK is already supported in radeon. Support for CIK in amdgpu
-	  will be disabled by default and is still provided by radeon.
-	  Use module options to override this:
+	  CIK (Sea Islands) are second generation GCN GPUs,
+	  supported by both drivers: radeon (old) and amdgpu (new).
+	  By default,
+	  CIK dedicated GPUs are supported by amdgpu
+	  CIK APUs are supported by radeon
 
+	  Use module options to override this:
+	  To use amdgpu for CIK,
 	  radeon.cik_support=0 amdgpu.cik_support=1
+	  To use radeon for CIK,
+	  radeon.cik_support=1 amdgpu.cik_support=0
 
 config DRM_AMDGPU_USERPTR
 	bool "Always enable userptr write support"