drm/amdgpu: Initialize asic functions for Arcturus

After cherry-picking doorbell rework changes from drm-next
branch, Arcturus asic functions pointer need to be initialized
to init doorbell index for Arcturus.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/soc15.c

@@ -1012,6 +1012,7 @@ static int soc15_common_early_init(void *handle)
 				AMD_PG_SUPPORT_RLC_SMU_HS;
 		break;
 	case CHIP_ARCTURUS:
+		adev->asic_funcs = &vega20_asic_funcs;
 		adev->cg_flags = 0;
 		adev->pg_flags = 0;
 		break;