drm/i915/gem: Optionally set SSEU in intel_context_set_gem

For now this is a no-op because everyone passes in a null SSEU but it
lets us get some of the error handling and selftest refactoring plumbed
through.

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/20210708154835.528166-19-jason@jlekstrand.net