drm/etnaviv: move MMU context ref/unref into map/unmap_gem

This makes it a little more clear that the mapping holds a reference
to the context once the buffer has been successfully mapped into that
context and simplifies the error handling a bit.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Guido Günther <agx@sigxcpu.org>