drm/omap: use dma_mapping_error in omap_gem_attach_pages

omap_gem_attach_pages() 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_attach_pages().

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