drm: fix compile failure by including <linux/swiotlb.h>

On tile architecture (with "make allyesconfig") including
<linux/swiotlb.h> is required to call swiotlb_nr_tbl().

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>