drm/nouveau/kms: switch to drm fbdev helpers

This removes support for accelerated fbcon rendering, and fixes a number
of races/crashes/issues around suspend/resume/module unload etc.

Losing HW accelerated rendering isn't ideal, but it's been significantly
reduced in performance since the removal of accelerated scrolling in the
kernel anyway - not to mention, can be racey (skips cpu<->gpu sync) from
certain contexts.

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