drm/i915: ibx_write_infoframe can disable AVI

IBX does not need the workaround used in cpt_write_infoframe that
requires the AVI frame to be enabled while being updated.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>