drm: Used DRM_LEGACY for all legacy functions

Except for nouveau, only legacy drivers need this really. And nouveau
is already marked up with DRIVER_KMS_LEGACY_CONTEXT as the special
case.

I've tried to be careful to leave everything related to modeset still
using the DRIVER_MODESET flag. Otherwise it's a direct replacement of
!DRIVER_MODESET with DRIVER_LEGACY checks. Also helps readability
since fewer negative checks overall.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470251470-30830-2-git-send-email-daniel.vetter@ffwll.ch