drm/ofdrm: Cast error pointers to void __iomem *

Cast error pointers when returning them as void __iomem *. Fixes
a number of Sparse warnings, such as the ones shown below.

../drivers/gpu/drm/tiny/ofdrm.c:439:31: warning: incorrect type in return expression (different address spaces)
../drivers/gpu/drm/tiny/ofdrm.c:439:31:    expected void [noderef] __iomem *
../drivers/gpu/drm/tiny/ofdrm.c:439:31:    got void *
../drivers/gpu/drm/tiny/ofdrm.c:442:31: warning: incorrect type in return expression (different address spaces)
../drivers/gpu/drm/tiny/ofdrm.c:442:31:    expected void [noderef] __iomem *
../drivers/gpu/drm/tiny/ofdrm.c:442:31:    got void *

See [1] for the bug report.

v3:
	* use IOMEM_ERR_PTR() (Javier)

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/dri-devel/202210200016.yiQzPIy0-lkp@intel.com/ # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20221103101627.32502-3-tzimmermann@suse.de