misc/emacs: Enable compilation-mode for gofmt error buffer before displaying it.

Some packages, like popwin.el, change display behaviour based on
the buffer's mode, so we should enable compilation-mode before
displaying the buffer.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8155043

Committer: Brad Fitzpatrick <bradfitz@golang.org>