i965: Pass a shader stage abbreviation to fs_generator().

i965: Pass a shader stage abbreviation to fs_generator().

A lot of messages hardcoded the string "FS", which is confusing on
Broadwell, where we use this code for VS support as well.

shader-db particularly got confused, as it reported two "FS SIMD8"
shaders, and no vertex shaders at all.  Craziness ensued.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>