drm/radeon/kms: Fix r600 blit cleanup path

r600 blit cleanup path need to check if a bo was allocated before
trying to free or unpin it. This patch add this check and avoid
oops when the initialization on r6xx or r7xx hw fails.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/rv770.c b/drivers/gpu/drm/radeon/rv770.c
index 16f7317..7f17ddf 100644
--- a/drivers/gpu/drm/radeon/rv770.c
+++ b/drivers/gpu/drm/radeon/rv770.c
@@ -975,10 +975,12 @@
 	r600_wb_disable(rdev);
 	rv770_pcie_gart_disable(rdev);
 	/* unpin shaders bo */
-	r = radeon_bo_reserve(rdev->r600_blit.shader_obj, false);
-	if (likely(r == 0)) {
-		radeon_bo_unpin(rdev->r600_blit.shader_obj);
-		radeon_bo_unreserve(rdev->r600_blit.shader_obj);
+	if (rdev->r600_blit.shader_obj) {
+		r = radeon_bo_reserve(rdev->r600_blit.shader_obj, false);
+		if (likely(r == 0)) {
+			radeon_bo_unpin(rdev->r600_blit.shader_obj);
+			radeon_bo_unreserve(rdev->r600_blit.shader_obj);
+		}
 	}
 	return 0;
 }