drm/nouveau: fix incorrect FB_BACKLIGHT usage in Kconfig

Making FB_BACKLIGHT tristate by commit b4a1ed0c ("fbdev:
make FB_BACKLIGHT a tristate") caused unmet dependencies in
some configurations:

WARNING: unmet direct dependencies detected for FB_BACKLIGHT
  Depends on [m]: HAS_IOMEM [=y] && FB [=m]
  Selected by [y]:
  - DRM_NOUVEAU [=y] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && MMU [=y] && DRM_NOUVEAU_BACKLIGHT [=y]
  Selected by [m]:
  - FB_NVIDIA [=m] && HAS_IOMEM [=y] && FB [=m] && PCI [=y] && FB_NVIDIA_BACKLIGHT [=y]

Fix it by making DRM_NOUVEAU select BACKLIGHT_CLASS_DEVICE and
BACKLIGHT_LCD_SUPPORT instead of FB_BACKLIGHT.

Fixes: b4a1ed0c

 ("fbdev: make FB_BACKLIGHT a tristate")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Cc: Rob Clark <robdclark@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>