drm/nv50-/disp: Use output specific mask in interrupt

The commit

   commit 476e84e1
   Author: Ben Skeggs <bskeggs@redhat.com>
   Date:   Mon Feb 11 09:24:23 2013 +1000

       drm/nv50-/disp: initial supervisor support for off-chip encoders

changed the write mask in one of the interrupt functions for on-chip encoders,
causing a regression in certain VGA dual-head setups. This commit reintroduces
the mask thus resolving the regression

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66129


Reported-and-Tested-by: Yves-Alexis <corsac@debian.org>
Cc: stable@vger.kernel.org [3.9+]
CC: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>