m68k/atari - atafb: convert allocation of fb ram to new interface

The new atari_stram_alloc interface returns kernel virtual addresses
even if the kernel runs in FastRAM. These addresses are not
guaranteed to be identical with the physical addresses. Since ST-RAM
mappings have not been set up by mem_init, virt_to_phys() and its
cousin do not work and the atari_stram_to_phys() etc. helpers must
be used to determine physical addresses.

fb.fix->smem_start needs physical addresses, fb.par->screen_base
needs virtual addresses. Take care of the virt-to-phys conversion
both on fb init and par changes.

Signed-off-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>