drm/etnaviv: return context from etnaviv_iommu_context_get

commit 78edefc05e41352099ffb8f06f8d9b2d091e29cd upstream.

Being able to have the refcount manipulation in an assignment makes
it much easier to parse the code.

Cc: stable@vger.kernel.org # 5.4
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Michael Walle <michael@walle.cc>
Tested-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>