drm/drv: DOC: Add driver example code

Add driver example that shows how devm_drm_dev_init() can be used.

v2: Expand docs (Sam, Daniel)

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225144232.20761-4-noralf@tronnes.org