gma500: remove references to drm_display_info raw_edid field

Another reference to raw_edid field of struct drm_display_info was added in
gma500 while the whole field was being removed, causing build
failure. Remove the hopefully last references to raw_edid.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>