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

drivers/gpu/drm/amd/amdgpu/Kconfig

···
 	bool "Enable amdgpu support for SI parts"
 	depends on DRM_AMDGPU
 	help
-	  Choose this option if you want to enable experimental support
+	  Choose this option if you want to enable support
 	  for SI (Southern Islands) asics.
 
-	  SI is already supported in radeon. Experimental support for SI
-	  in amdgpu will be disabled by default and is still provided by
-	  radeon. Use module options to override this:
+	  SI (Southern Islands) are first generation GCN GPUs,
+	  supported by both drivers: radeon (old) and amdgpu (new).
+	  By default, SI dedicated GPUs are supported by amdgpu.
 
-	  radeon.si_support=0 amdgpu.si_support=1
+	  Use module options to override this:
+	  To use radeon for SI,
+	  radeon.si_support=1 amdgpu.si_support=0
 
 config DRM_AMDGPU_CIK
 	bool "Enable amdgpu support for CIK parts"

drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c

···
  * SI (Southern Islands) are first generation GCN GPUs, supported by both
  * drivers: radeon (old) and amdgpu (new). This parameter controls whether
  * amdgpu should support SI.
- * By default, SI chips are supported by radeon (except when radeon is not built).
+ * By default, SI dedicated GPUs are supported by amdgpu.
  * Only relevant when CONFIG_DRM_AMDGPU_SI is enabled to build SI support in amdgpu.
  * See also radeon.si_support which should be disabled when amdgpu.si_support is
  * enabled, and vice versa.
···
 		param = "si_support";
 		module_param = amdgpu_si_support;
 		amdgpu_support_built = IS_ENABLED(CONFIG_DRM_AMDGPU_SI);
+		support_by_default = true;
 		break;
 
 	case CHIP_BONAIRE:

drivers/gpu/drm/radeon/radeon_drv.c

···
 		gen = "SI";
 		module_param = radeon_si_support;
 		amdgpu_support_built &= IS_ENABLED(CONFIG_DRM_AMDGPU_SI);
+		support_by_default = false;
 		break;
 
 	case CHIP_BONAIRE: