drm/omap: use refcount API to track the number of users of dma_addr

This would give us a WARN_ON() if the pin/unpin calls are unbalanced.

Proposed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-2-jjhiblot@ti.com