drm/i915: Deduplicate the code to fill the aux message header

drm/i915: Deduplicate the code to fill the aux message header

We have two instances of the code to fill out the header for the aux
message. Pull it into a small helper.

v2: Rebase due to txbuf[] changes

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/20180222212802.4826-1-ville.syrjala@linux.intel.com
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>