drm/mgag200: Enable caching for SHMEM pages

SHMEM pages use write-combine caching by default, but can also use the
platform's default page caching. Doing so may improve the performance
of I/O on the framebuffer.

Mgag200's hardware does not access framebuffer pages directly (i.e.,
via DMA), so enabling caching does not have an effect on consistency
of the framebuffer memory or the displayed data.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-2-tzimmermann@suse.de