drm/gma500: Clear fbdev framebuffer with fb_memset_io()

The fbdev framebuffer is I/O memory, so clear it with fb_memset_io().
Fixes the following sparse warning:

../drivers/gpu/drm/gma500/fbdev.c:234:20: warning: incorrect type in argument 1 (different address spaces)
../drivers/gpu/drm/gma500/fbdev.c:234:20:    expected void *s
../drivers/gpu/drm/gma500/fbdev.c:234:20:    got char [noderef] __iomem *screen_base

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230522190637.7039-1-tzimmermann@suse.de