drm/gma500: Don't destroy DRM properties in the driver

When drm properties are created, they are added to mode_config.property_list
which is then used in drm_mode_config_cleanup() to destroy every single
property created by the driver.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>