nir/lower_fragcolor: Fix driver_location assignment
Fixes crash in dEQP-GLES31.functional.shaders.framebuffer_fetch.basic.last_frag_data when using this pass. Signed-off-by:Alyssa Rosenzweig <alyssa@collabora.com> Reviewed-by:
Erik Faye-Lund <erik.faye-lund@collabora.com> Part-of: <mesa/mesa!10411> (cherry picked from commit 73eb497b)