drm/nv04-nv40: Disable connector polling when there're no spare CRTCs left.

Load detection needs the connector wired to a CRTC, when there are no
inactive CRTCs left that means we need to cut some other head off for
a while, causing intermittent flickering.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>