drm/i915/gem: Return an error ptr from context_lookup

We're about to start doing lazy context creation which means contexts
get created in i915_gem_context_lookup and we may start having more
errors than -ENOENT.

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/20210708154835.528166-23-jason@jlekstrand.net