drm/omap: return error if dma_alloc_writecombine fails

drm/omap: return error if dma_alloc_writecombine fails

On a platform with no TILER (e.g. omap3, am43xx), when the user wants to
allocate buffer with flag OMAP_BO_SCANOUT, the buffer needs to be
allocated with dma_alloc_writecombine. For some reason the driver does
not return an error if that alloc fails, instead it continues without
backing memory. This leads to errors later when the user tries to use
the buffer.

This patch makes the driver return an error if dma_alloc_writecombine
fails.

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