drm/i915: avoid hanging on to a stale pointer to raw_edid.

drm_get_edid will store edid into raw_edid, so when freeing edid memory,
at the same time clean raw_edid pointer.

Signed-off-by: Ma Ling <ling.ma@intel.com>
[anholt: Note that raw_edid is not currently used anywhere]
Signed-off-by: Eric Anholt <eric@anholt.net>