drm/exynos: don't use DMA_ERROR_CODE

DMA_ERROR_CODE already isn't a valid API to user for drivers and will
go away soon.  exynos_drm_fb_dma_addr uses it a an error return when
the passed in index is invalid, but the callers never check for it
but instead pass the address straight to the hardware.

Add a WARN_ON instead and just return 0.

Signed-off-by: Christoph Hellwig <hch@lst.de>