drm/radeon: add new getparam for number of backends.

This allows userspace to work out how many DBs there are
for conditional rendering to work.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon_kms.c b/drivers/gpu/drm/radeon/radeon_kms.c
index 4057ebf..68a7b22 100644
--- a/drivers/gpu/drm/radeon/radeon_kms.c
+++ b/drivers/gpu/drm/radeon/radeon_kms.c
@@ -205,6 +205,17 @@
 		/* return clock value in KHz */
 		value = rdev->clock.spll.reference_freq * 10;
 		break;
+	case RADEON_INFO_NUM_BACKENDS:
+		if (rdev->family >= CHIP_CEDAR)
+			value = rdev->config.evergreen.max_backends;
+		else if (rdev->family >= CHIP_RV770)
+			value = rdev->config.rv770.max_backends;
+		else if (rdev->family >= CHIP_R600)
+			value = rdev->config.r600.max_backends;
+		else {
+			return -EINVAL;
+		}
+		break;
 	default:
 		DRM_DEBUG_KMS("Invalid request %d\n", info->request);
 		return -EINVAL;