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>