drm/mgag200: Fix caching setup for remapped video memory

I/O video memory for the framebuffer supports write-combine caching
mode. Simplify the driver's code that sets up the caching mode.

 * Map video memory with ioremap_wc(), which automatically sets up
the PAT entry with write-combine caching.

 * Remove the now obsolete call to devm_arch_io_reserve_memtype_wc().
It is only required to mmap the video memory to user space, which the
driver doesn't do.

 * According to the PAT documentation, arch_phys_wc_add() is best
called after remapping I/O memory, so move it after ioremap.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240105082714.21881-1-tzimmermann@suse.de