viafb: clean up virtual memory handling

Clean the handling of ioremapped video memory up.  The following changes
were made:

info->screen_base - viafb_FB_MM
(VRAM offset calculation) was replaced by
info->fix.smem_start - viafbinfo->fix.smem_start
which is essentially the same calculation but done with physical instead
virtual addresses.

*->fbmem_virt
was replaced by
viafbinfo->screen_base
This is true for viafbinfo and viafbinfo1 as the par pointers are equal.

An early initialization of viafbinfo1->fix.smem* was removed as done later
in viafb_setup_fixinfo.

This patch highlights that the only usage of the ioremapped video memory
in the driver is for hardware cursor handling.  Even if it has to hold the
used virtual screen mapped for old-fashioned read/write calls (vs.
mmap'ed) a lot virtual memory could be saved by only ioremapping on
demand.

Code cleanup, no runtime changes expected.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Harald Welte <laforge@gnumonks.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>