fbdev/vermillion: use vm_iomap_memory()

Use vm_iomap_memory() instead of [io_]remap_pfn_range().
vm_iomap_memory() gives us much simpler API to map memory to userspace,
and reduces possibilities for bugs.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Alan Hourihane <alanh@fairlite.demon.co.uk>