drm: Mark up legacy/dri1 drivers with DRM_LEGACY

drm: Mark up legacy/dri1 drivers with DRM_LEGACY

It's super confusing that new drivers need to be marked with
DRIVER_MODESET when really it means DRIVER_MODERN. Much better to
invert the meaning and rename it to something that's suitably
off-putting.

Since there's over 100 places using DRIVER_MODESET we need to roll out
this change without a flag day.

v2: Update docs.

Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470251470-30830-1-git-send-email-daniel.vetter@ffwll.ch