drm/i915: Introduce a get_aux_send_ctl() vfunc

We need a bit more flexibility here in the future, bits get shuffled
around.

v2: more descriptive commit message (Jani Nikula)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>