fb: fix screen pixmap leak on server reset

Call FreePixmap() instead of free() to destroy the screen pixmap in
fbCloseScreen().

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

fb/fbscreen.c

@@ -37,7 +37,7 @@ fbCloseScreen(ScreenPtr pScreen)
         free(depths[d].vids);
     free(depths);
     free(pScreen->visuals);
-    free(pScreen->devPrivate);
+    FreePixmap((PixmapPtr)pScreen->devPrivate);
     return TRUE;
 }