drm/gma500: Split GTT init/resume/fini into GTT and GEM functions

The GTT init, fini and resume functions contain both, GTT and GEM,
code. Split each into a separate GTT and a GEM function. The GEM
code is responsible for mmap_mutex and the stolen memory area. The
rest of the functionality is left in GTT functions.

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/20220308195222.13471-9-tzimmermann@suse.de