drm/i915/gem: Add a separate validate_priority helper

With the proto-context stuff added later in this series, we end up
having to duplicate set_priority.  This lets us avoid duplicating the
validation logic.

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/20210708154835.528166-15-jason@jlekstrand.net