drm/nv50/gpio: post-nv92 cards have 32 interrupt lines

Since the original merge of nouveau to upstream kernel, we were assuming
that nv90 (and later) cards have 32 lines.

Based on mmio traces of the binary driver, as well as PBUS error messages
during read/write of the e070/e074 registers, we can conclude that nv92
has only 16 lines whereas nv94 (and later) cards have 32.

Reported-and-tested-by: David M. Lloyd <david.lloyd@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>