drm/nouveau/gr/gf100-: split out per-gpc address calculation macro

There's a few places where we need to access a GPC register from ucode,
but outside of the falcon's io address space.  To do this we need to
calculate the offset based on which GPC we're executing on.

This used to be done manually, but we've since found a "base" offset
that can be added by the hardware.  To use this, an extra bit needs to
be set in the register address, which is what this macro achieves.

There should be no functional change from this commit.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>