i965/fs: Special-case nir_intrinsic_store_output for the fragment shader.

i965/fs: Special-case nir_intrinsic_store_output for the fragment shader.

I'm about to change how fragment shader output locations are
represented, so the generic nir_intrinsic_store_output implementation
that assumes that outputs are just contiguous elements in the big
nir_outputs array won't work anymore.  This somewhat simplified
implementation of nir_intrinsic_store_output for fragment shaders
should be functionally equivalent to the current fall-back one.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>