drm/simpledrm: Use struct iosys_map consistently

The majority of the driver already uses struct iosys_map to encapsulate
accesses to I/O remapped vs. system memory. Accesses via the screen base
pointer still use __iomem annotations, which can lead to inconsistencies
and conflicts with subsequent patches.

Convert the screen base to a struct iosys_map as well for consistency
and to avoid these issues.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-5-thierry.reding@gmail.com