drm: omapdrm: DMA-unmap pages for all buffer types when freeing buffers

Both coherent (uncached) and non-coherent (cached) buffers can have
their pages mapped to the device through the DMA mapping API. Make sure
to unmap any mapped page when freeing a buffer, regardless of its type.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>