i965/fs: Fix regression in fbo-alphatest-nocolor.
i965/fs: Fix regression in fbo-alphatest-nocolor.

In the refactor for handling user-defined out params, we failed to set
up the new color output tracking when there was no color drawbuffer in
place but alpha testing was on.  Just always set up at least one when
handling gl_FragColor, since we won't make use of its value unless we
need to.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42806

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