drm/etnaviv: move cmdbuf into submit object

Less dynamic allocations and slims down the cmdbuf object to only the
required information, as everything else is already available in the
submit object.

This also simplifies buffer and mappings lifetime management, as they
are now exlusively attached to the submit object and not additionally
to the cmdbuf.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>