drm/omap: omap_gem: Do not try to unmap page which is not mapped

It might be possible that the page has been unmapped already in
omap_gem_cpu_sync() so check before calling dma_unmap_page().

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>