drm/qxl: drop qxl_ttm_fault

Not sure what this hook is supposed to do.  vmf->vma->vm_private_data
should never be NULL, so the extra check in qxl_ttm_fault should have no
effect.

Drop it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191017132638.9693-2-kraxel@redhat.com