drm: Some ocd in drm_file.c

Move the open helper around to avoid the forward decl, and give
drm_setup a drm_legacy_ prefix since it's all legacy stuff in there.

v2: Move drm_legacy_setup into drm_legacy_misc.c (Chris). The
counterpart in the form of drm_legacy_dev_reinit is there already too,
plus it fits perfectly into Dave's work of making DRIVER_LEGACY code
compile-time optional.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190502135603.20413-1-daniel.vetter@ffwll.ch