drm/gma500: Remove dependency on TTM

The gma500 driver does not use TTM.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210129095604.32423-6-tzimmermann@suse.de