drm/amdgpu: Use amdgpu by default on SI dedicated GPUs (v2)

drm/amdgpu: Use amdgpu by default on SI dedicated GPUs (v2)

Now that the DC analog connector support and VCE1 support landed,
amdgpu is at feature parity with the old radeon driver
on SI dGPUs.

Enabling the amdgpu driver by default for SI 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.si_support=0 radeon.si_support=1

v2:
- Update documentation in Kconfig file

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>