nv50/ir: make use of auxCBSlot instead of magic numbers

nv50/ir: make use of auxCBSlot instead of magic numbers

This avoids using magic numbers for the driver constbuf slot which
is always 15 except for compute shaders on gk104+ where the slot 0
is used.

For gk104+, some special compute-related values like the thread
index are uploaded to screen->parm which is currently bound on c0.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Pierre Moreau <pierre.morrow@free.fr>