i915: Fix memory leak in do_blit_readpixels.

Fixes "Resource leak" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i915/intel_pixel_read.c

@@ -148,6 +148,7 @@ do_blit_readpixels(struct gl_context * ctx,
                            pbo_mt, 0, 0,
                            0, 0, dst_flip,
                            width, height, GL_COPY)) {
+      intel_miptree_release(&pbo_mt);
       return false;
    }