nv50: fix layerid to be the fp input number rather than vp output number

nv50: fix layerid to be the fp input number rather than vp output number

In the tests they were the same so it didn't matter, but indications are
that this is the correct behaviour. Also take this opportunity to
(trivially) support using gl_Layer in fp.

Cc: 10.1 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Christoph Bumiller <e0425955@student.tuwien.ac.at>