drm/nouveau/disp/nv04-nv40: abort scanoutpos query on vga analog.

nv04_disp_scanoutpos() must abort to trigger simple timestamping
fallback if vtotal/htotal regs return zero. This happens if the
output isn't a digital output, but a vga analog output, as the
regs don't get initialized in that case.

Fixes timestamping failure on nv-40 and earlier with vga output.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: <stable@vger.kernel.org> # 3.14+