drm/nouveau/gpio: rename g92 class to g94

nv92 hardware has only 16 interrupt lines, while nv94 and later
has 32. Accessing 0xe0c{0,4} registers on nv92 can lead to incorrect
PDISP setup. This is a regression introduced with

commit 9d0f5ec9ee0fd5dc5fc1cc2cf559286431e406e3
Author: Ben Skeggs <bskeggs@redhat.com>
Date:   Mon May 12 15:22:42 2014 +1000

    gpio: split g92 class from nv50

Reported-by: estece on #nouveau
Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>