drm/i915: Factor out a function returning the AUX_CTL value to start a send

Also, move that computation outside of the for loop that tries 5 times,
this value doesn't change between tries.

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