drm/meson: Use drm_dev_put() instead of drm_dev_unref()

According to 'drivers/gpu/drm/drm_drv.c', 'drm_dev_unref()' is just a
compatibility alias for 'drm_dev_put()'. So use the latter instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d4ce46077d3ada7149c4555acfdda8ade4a6cf50.1520885192.git.christophe.jaillet@wanadoo.fr