st/mesa: unmap pbo after updating cache

Unmapping first leads to accessing an invalid pointer. So let's switch
these lines around.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_cb_drawpixels.c

@@ -722,12 +722,12 @@ make_texture(struct st_context *st,
       ctx->_ImageTransferState = imageTransferStateSave;
    }
 
-   _mesa_unmap_pbo_source(ctx, unpack);
-
 #if USE_DRAWPIXELS_CACHE
    cache_drawpixels_image(st, width, height, format, type, unpack, pixels, pt);
 #endif
 
+   _mesa_unmap_pbo_source(ctx, unpack);
+
    return pt;
 }