drm/gma500: update comment of psb_spank()

In commit 5c209d80 ("drm/gma500: psb_spank() doesn't need it's
own file"), accel_2d.c was deleted and psb_spank() was moved into
psb_drv.c. Fix the comment here.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210506112851.20315-1-wanjiabing@vivo.com