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/r600.c b/drivers/gpu/drm/radeon/r600.c
index 9757962..3e02dd4 100644
--- a/drivers/gpu/drm/radeon/r600.c
+++ b/drivers/gpu/drm/radeon/r600.c
@@ -1961,11 +1961,13 @@
 	r600_wb_disable(rdev);
 	r600_pcie_gart_disable(rdev);
 	/* unpin shaders bo */
-	r = radeon_bo_reserve(rdev->r600_blit.shader_obj, false);
-	if (unlikely(r != 0))
-		return r;
-	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 (!r) {
+			radeon_bo_unpin(rdev->r600_blit.shader_obj);
+			radeon_bo_unreserve(rdev->r600_blit.shader_obj);
+		}
+	}
 	return 0;
 }