drivers/gpu/drm/i915/i915_gem.c: Add missing error handling code

Extend the error handling code with operations found in other nearby error
handling code

A simplified version of the sematic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r exists@
@r@
statement S1,S2,S3;
constant C1,C2,C3;
@@

*if (...)
 {... S1 return -C1;}
...
*if (...)
 {... when != S1
    return -C2;}
...
*if (...)
 {... S1 return -C3;}
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org

drivers/gpu/drm/i915/i915_gem.c

@@ -3258,6 +3258,8 @@ i915_gem_object_pin_and_relocate(struct drm_gem_object *obj,
 				  (int) reloc->offset,
 				  reloc->read_domains,
 				  reloc->write_domain);
+			drm_gem_object_unreference(target_obj);
+			i915_gem_object_unpin(obj);
 			return -EINVAL;
 		}
 		if (reloc->write_domain & I915_GEM_DOMAIN_CPU ||