drm: Update docs for legacy kms state

Point at the equivalent atomic state and explain that atomic drivers
shouldn't really depend upon legacy state.

Motivated by questions from Manasi about how this all is supposed to
work.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171108203007.12274-1-daniel.vetter@ffwll.ch