i965/vs: Gen4/5: enable front colors if back colors are written

i965/vs: Gen4/5: enable front colors if back colors are written

Fixes undefined results if a back color is written, but the
corresponding front color is not, and only backfacing primitives are
drawn. Results are still undefined if a frontfacing primitive is drawn,
but that's OK.

The other reasonable way to fix this would have been to just pick
the one color slot that was populated, but that dilutes the value of
the tests.

On Gen6+, the fixed function clipper and triangle setup already take
care of this.

Fixes 11 piglits:
spec/glsl-1.10/execution/interpolation/interpolation-none-gl_Back*Color-*

NOTE: This is a candidate for stable branches.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>