drm/fb-helper: Look up preferred fbdev node number from DT

For situations where there are multiple DRM cards in a system,
add a query of DT for "drm_fb" designations for cards to set
their preferred /dev/fbN designation.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

drm/fb_helper: Change query for FB designation from drm_fb to drm-fb

Fixes: 1216ea56 ("drm/fb-helper: Look up preferred fbdev node number from DT")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>