drm/nouveau: do not forcibly power on lvds panels

This fix was put in place to fix a bug where the eDP panel on certain
laptops fails to respond over the aux channel after suspend.

It appears that on some systems (Dell M6600, with LVDS panel) there's a
very bad interaction with the eDP init table that causes the SOR to get
very confused and not drive the panel correctly, leading to bleed.

A DPMS off/on cycle is enough to bring it back, but, this will avoid the
problem by not touching the panel GPIOs at times we're not meant to.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>