drm/vmwgfx: Stop checking for DRM_UNLOCKED

drm core enforces now for DRIVER_MODESET that all ioctls are unlocked.
And all the old nasty ones from drm core aren't allowed for modern
drivers any more. Hence this is no longer needed.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>