drm/fb-helper: Prepare to move out modeset config code

This prepares the modeset code so it can be moved out as-is in the next
patch.

v3: Remove stray newline

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/20190608152657.36613-3-noralf@tronnes.org