drm/nv04/crtc: Bail out if FB is not bound to crtc

This commit resolves a possible 'NULL pointer dereference'
It uses the same approach as radeon, intel and nouveau/nv50

Fixes bug 'Nouveau: Kernel oops when unplugging external monitor'
https://bugs.freedesktop.org/show_bug.cgi?id=40336



Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>