isl: Drop UnormPathInColorPipe for buffer surfaces.

Jason suggested I remove this in review, and he's right.  AFAICT this
affects blending, and that just isn't going to happen on buffers.

Fixes: f741de23 ("isl: Enable Unorm Path in Color Pipe")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 1b090f06)