drm/gma500: Inline psb_gtt_attach_pages() and psb_gtt_detach_pages()

psb_gtt_attach_pages() are not GTT functions but deal with the GEM
object's SHMEM pages. The only callers of psb_gtt_attach_pages() and
psb_gtt_detach_pages() are the GEM pin helpers. Inline the calls and
cleanup the resulting code.

v2:
	* unlock gtt_mutex in pin-error handling (Patrik)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-7-tzimmermann@suse.de