drm/gma500: Set page-caching flags in GEM pin/unpin

Caching of the GEM object's backing pages are unrelated to GTT
management. Move the respective calls from GTT code to GEM code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-9-tzimmermann@suse.de