drm/fb-helper: Move out commit code

Move the modeset commit code to drm_client_modeset.
No changes except exporting API.

v7: Export drm_client_panel_rotation() (Gerd Hoffmann)
v2: Move to drm_client_modeset.c instead of drm_client.c

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190531140117.37751-5-noralf@tronnes.org