drm/i915/gem: Return void from context_apply_all

None of the callbacks we use with it return an error code anymore; they
all return 0 unconditionally.

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/20210708154835.528166-6-jason@jlekstrand.net