drm/atomic-helper: Move DRM_PLANE_HELPER_NO_SCALING to atomic helpers

The macro DRM_PLANE_HELPER_NO_SCALING is only useful with the interfaces
in drm_atomic_helper.h, but defined in drm_plane_helper.h. So half of
DRM includes the latter header file for using this macro. Move the macro
and remove the include statements.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220720083058.15371-3-tzimmermann@suse.de