drm/gma500: Move GTT locking into GTT helpers

Acquire the GTT mutex in psb_gtt_{insert,remove}_pages(). Remove
locking from callers. Also remove the GTT locking around the resume
code. Resume does not run concurrently with other GTT operations.

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/20220308195222.13471-4-tzimmermann@suse.de