drm/omap: use dma_mapping_error in omap_gem_dma_sync

omap_gem_dma_sync() calls dma_map_page() but does not check the possible
error with dma_mapping_error(). If DMA-API debugging is enabled, the
debug layer will give a warning if dma_mapping_error() has not been
used.

This patch adds proper error handling to omap_gem_dma_sync().

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