drm/nouveau: Refactor nvXX_backlight_init()

There's literally no difference between any of the backlight init
functions besides the backlight properties they set and the backlight
callbacks that they set, so move all of the duplicated backlight init
code out of there and into nouveau_backlight_init().

This gets rid of a lot of copy pasta!

Changes since v1:
- Some of the pre-refactor callbacks were storing nv_encoder in callback
  data for the backlight devices that they registered, as opposed to
  nouveau_drm. This got missed and caused some bugs that didn't
  originally appear on my setup (NULL kernel derefs) for some reason.
  So, fix this by finding the nouveau_encoder in
  nouveau_backlight_init(), and using that as the callback data for all
  gens instead even if they don't care about the encoder.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jeffery Miller <jmiller@neverware.com>
Cc: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>