video: ocfb: Fix data type warning

When allocate framebuffer memory using dma_alloc_coherent(),
we'd better use dma_addr_t instead of phys_addr_t. Because the
address we got in fact is DMA or bus address for the platform.

This patch also fixes below build warning:
drivers/video/fbdev/ocfb.c:335:2:
	warning: passing argument 3 of ‘dma_alloc_attrs’
	from incompatible pointer type [enabled by default]

Signed-off-by: Qiang Chen <qiang2.chen@sonymobile.com>
Acked-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>