drm/gma500: Remove struct psb_gtt.sem sempahore

The semaphore at struct psb_mmu_driver.sem protects access to the MMU
fields. Additional locking with struct psb_gtt.sem is unnecessary. Remove
the field and related code.

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/20220308195222.13471-5-tzimmermann@suse.de