drm/i915/debugfs: stop using edid_blob_ptr

Only the EDID code and sysfs should look at the EDID property. Stop
using it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230602132321.3199445-1-jani.nikula@intel.com