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

drivers/gpu/drm/gma500/psb_drv.h

@@ -646,7 +646,7 @@ extern u32 psb_get_vblank_counter(struct drm_crtc *crtc);
 extern int psbfb_probed(struct drm_device *dev);
 extern int psbfb_remove(struct drm_device *dev,
 			struct drm_framebuffer *fb);
-/* accel_2d.c */
+/* psb_drv.c */
 extern void psb_spank(struct drm_psb_private *dev_priv);
 
 /* psb_reset.c */