drm/fb-helper: Move out modeset config code

No functional changes, just moving code as-is and fixing includes.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190608152657.36613-4-noralf@tronnes.org