drm/gma500: Rename psb_gtt_{pin,unpin}() to psb_gem_{pin,unpin}()

Rename psb_gtt_pin() to psb_gem_pin() to reflect the semantics of the
function. Same for psb_gtt_unpin(). No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-6-tzimmermann@suse.de