drm/i915/skl: Provide a get_aux_send_ctl() vfunc for skylake

Skylake doesn't use the pre-charge field now, but, instead, we need to
specify the total number of SYNC pulses for the SYNC phase (pre-charge +
SYNC pattern pules). Let's use the default value (32) for that.

v3: increase DP AUX TX timeout as 400us is not to be used on SKL
    apparently (Jesse).

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>