drm/nouveau/disp/mcp7x: disable dptmds workaround

The workaround appears to cause regressions on these boards, and from
inspection of RM traces, NVIDIA don't appear to do it on them either.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Roy Spliet <nouveau@spliet.org>