drm: Define enum mode_set_atomic in drm_modeset_helper_tables.h

Define enum mode_set_atomic next to the only interface that uses
the type. This will allow for removing several include statements
for drm_fb_helper.h. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111130206.29974-7-tzimmermann@suse.de