drm/ast: Init iosys_map pointer as I/O memory for damage handling

Ast hardware scans out the primary plane from video memory, which
is in I/O-memory space. Hence init the damage handler's iosys_map
pointer as I/O memory.

Not all platforms support accessing I/O memory as system memory,
although it's usually not a problem in ast's x86-based systems.

The error report is at [1].

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Fixes: f2fa5a99 ("drm/ast: Convert ast to SHMEM")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://lore.kernel.org/lkml/202212170111.eInM0unS-lkp@intel.com/T/#u # 1
Link: https://patchwork.freedesktop.org/patch/msgid/20221216193005.30280-1-tzimmermann@suse.de