fbdev/ep93xx-fb: Alloc DMA memory from hardware device

Pass the hardware device to the DMA helpers dma_alloc_wc(), dma_mmap_wc()
and dma_free_coherent(). The fbdev device that is currently being used is
a software device and does not provide DMA memory.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-13-tzimmermann@suse.de