drm/docs: improve docs for drm_drv.c

Just a bit of drive-by reading:
- drm_dev_set_unique() is really the exception, make that clear.
- drm_dev_init() is the recomended approach.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111164048.29067-1-daniel.vetter@ffwll.ch