fbdev: sh_mobile_lcdc: use dma_mmap_coherent

fb_mmap() implemented in fbmem.c uses smem_start as the physical
address of the frame buffer.  In the sh_mobile_lcdc driver, the
smem_start is a dma_addr_t that is not a physical address when IOMMU is
enabled.  dma_mmap_coherent() maps the address correctly.

Signed-off-by: Hideki EIRAKU <hdk@igel.co.jp>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>