drm/amd/display: enable SI support in the Kconfig (v2)

[Why]
All DCE6 specific code changes are guarded by CONFIG_DRM_AMD_DC_SI Kconfig option

[How]
(v1) CONFIG_DRM_AMD_DC_SI configuration option is added, default setting is disabled

(v2) Hainan is not supported, description updated accordingly

Tested with HD7750 (Cape Verde) and HD7950 (Tahiti)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/Kconfig

@@ -31,6 +31,14 @@ config DRM_AMD_DC_HDCP
 	help
 	  Choose this option if you want to support HDCP authentication.
 
+config DRM_AMD_DC_SI
+	bool "AMD DC support for Southern Islands ASICs"
+	default n
+	help
+	  Choose this option to enable new AMD DC support for SI asics
+	  by default. This includes Tahiti, Pitcairn, Cape Verde, Oland.
+	  Hainan is not supported by AMD DC and it has no physical DCE6.
+
 config DEBUG_KERNEL_DC
 	bool "Enable kgdb break in DC"
 	depends on DRM_AMD_DC