drm/drv: drm_dev_unplug(): Move out drm_dev_put() call

This makes it possible to use drm_dev_unplug() with the upcoming
devm_drm_dev_init() which will do drm_dev_put() in its release callback.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David (ChunMing) Zhou <David1.Zhou@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208140103.28919-3-noralf@tronnes.org