drm_fbdev_generic_setup() was changed to be a void return, but the stub
was left returning 0.

./include/drm/drm_fb_helper.h: In function ‘drm_fbdev_generic_setup’:
./include/drm/drm_fb_helper.h:450:9: warning: ‘return’ with a value, in function returning void [-Wreturn-type]
./include/drm/drm_fb_helper.h:448:1: note: declared here
  448 | drm_fbdev_generic_setup(struct drm_device *dev, unsigned int preferred_bpp)

Fixes: 1aed9509 ("drm/fb-helper: Remove return value from drm_fbdev_generic_setup()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408212407.4309-1-chris@chris-wilson.co.uk