drm/gma500: psb_spank() doesn't need it's own file

Since everything else in accel_2d.c got removed we can move psb_spank()
into psb_drv.c where it is used.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201132617.1233-5-patrik.r.jakobsson@gmail.com