drm/omap: fix dma_addr refcounting

cec4fa75 ("drm/omap: use refcount API to
track the number of users of dma_addr") changed omap_gem.c to use
refcounting API to track dma_addr uses.  However, the driver only tracks
the refcounts for non-contiguous buffers, and the patch didn't fully
take this in account.

After the patch, the driver always decreased refcount in omap_gem_unpin,
instead of decreasing the refcount only for non-contiguous buffers. This
leads to refcounting mismatch.

As for the contiguous cases the refcount is never increased, fix this
issue by returning from omap_gem_unpin if the buffer being unpinned is
contiguous.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114080343.30704-1-tomi.valkeinen@ti.com
Fixes: cec4fa75 ("drm/omap: use refcount API to track the number of users of dma_addr")
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/gpu/drm/omapdrm/omap_gem.c

@@ -843,9 +843,13 @@ int omap_gem_pin(struct drm_gem_object *obj, dma_addr_t *dma_addr)
  */
 static void omap_gem_unpin_locked(struct drm_gem_object *obj)
 {
+	struct omap_drm_private *priv = obj->dev->dev_private;
 	struct omap_gem_object *omap_obj = to_omap_bo(obj);
 	int ret;
 
+	if (omap_gem_is_contiguous(omap_obj) || !priv->has_dmm)
+		return;
+
 	if (refcount_dec_and_test(&omap_obj->dma_addr_cnt)) {
 		ret = tiler_unpin(omap_obj->block);
 		if (ret) {