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This means that the proto-context needs to grow support for engine
configuration information as well as setparam logic.  Fortunately, we'll
be deleting a lot of setparam logic on the primary context shortly so it
will hopefully balance out.

There's an extra bit of fun here when it comes to setting SSEU and the
way it interacts with PARAM_ENGINES.  Unfortunately, thanks to
SET_CONTEXT_PARAM and not being allowed to pick the order in which we
handle certain parameters, we have think about those interactions.

v2 (Daniel Vetter):
 - Add a proto_context_free_user_engines helper
 - Comment on SSEU in the commit message
 - Use proto_context_set_persistence in set_proto_ctx_param

v3 (Daniel Vetter):
 - Fix a doc comment
 - Do an explicit HAS_FULL_PPGTT check in set_proto_ctx_vm instead of
   relying on pc->vm != NULL.
 - Handle errors for CONTEXT_PARAM_PERSISTENCE
 - Don't allow more resetting user engines
 - Rework initialization of UCONTEXT_PERSISTENCE

v4 (Jason Ekstrand):
 - Move hand-rolled initialization of UCONTEXT_PERSISTENCE to an
   earlier patch

v5 (Jason Ekstrand):
 - Move proto_context_set_persistence to this patch
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-22-jason@jlekstrand.net