drm/radeon: remove load callback from kms_driver

The ".load" callback in "struct drm_driver" is deprecated. In order to remove
the callback, we have to manually call "radeon_driver_load_kms" instead.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Wu Hoi Pok <wuhoipok@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>