drm/vmwgfx: Call vmw_driver_{load,unload}() before registering device

The load/unload callbacks in struct drm_driver are deprecated. Remove
them and call functions explicitly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>