drm/i915: Keep the AKSV details in intel_dp_hdcp_write_an_aksv()

Let's try to keep the details on the AKSV stuff concentrated
in one place. So move the control bit and +5 data size handling
there.

v2: Increase txbuf[] to include the payload which intel_dp_aux_xfer()
    will still load into the registers even though the hardware
    will ignore it

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222212732.4665-1-ville.syrjala@linux.intel.com


Reviewed-by: Ramalingam C <ramalingam.c@intel.com>