drm/i915: Simplify audio handling on DDI ports

There's no need to check whether audio is enabled (which for ddi ports
is done through the crtc->eld_vld flag) since at the cost of a
potentially unecessary register rmw cycle we can unconditionally do
this.

Note that the edp check is just paranoia since we won't ever call the
write_eld function for an edp panel.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>