drm/drv: Hold ref on parent device during drm_device lifetime

[ Upstream commit 56be6503aab2bc3a30beae408071b9be5e1bae51 ]

This makes it safe to access drm_device->dev after the parent device has
been removed/unplugged.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225144232.20761-2-noralf@tronnes.org


Signed-off-by: Sasha Levin <sashal@kernel.org>