drm/i915/skl: Implement the get_aux_clock_divider() DP vfunc

We need to provide a vfunc that will make the code in intel_dp_aux_ch()
loop once to start the AUX transaction. The return value (clock divider)
is unused on SKL, so just return 1.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>