drm/gma500: Move GEM memory management functions to gem.c

Move GEM functions from gtt.c to gem.c. Adapt some names. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220308195222.13471-11-tzimmermann@suse.de