drm/etnaviv: provide MMU context to etnaviv_gem_mapping_get

In preparation to having a context per process, etnaviv_gem_mapping_get
should not use the current GPU context, but needs to be told which
context to use.

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