i965: Don't perform the precompile on fragment shaders by default.

i965: Don't perform the precompile on fragment shaders by default.

It is useful to have this option for shader-db, and it was also good
at the time where we were rejecting shaders due to various internal
limits we hadn't supported yet.  However, at this point the precompile
step takes extra time (since not all NOS is known at link time) and
spews misleading debug in the common case of debugging a real app.

This is left in place for VS, where we still have a couple of codegen
failure paths that result in link failure through precompile.  Those
need to be fixed.

shader-db can still get at the debug info it wants using
"shader_precompile=true" driconf option.  Long term, we can probably
build a good-enough app for shader-db to trigger real codegen.