nv50,nvc0: make sure to destroy the mutex used for blits

This mutex is initialized when the blitter is created, but it is never
destroyed. This doesn't hurt anything but it makes sense to destroy it
at blitter deletion.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/nv50/nv50_surface.c

@@ -1756,6 +1756,7 @@ nv50_blitter_destroy(struct nv50_screen *screen)
       }
    }
 
+   pipe_mutex_destroy(blitter->mutex);
    FREE(blitter);
 }
 

src/gallium/drivers/nouveau/nvc0/nvc0_surface.c

@@ -1644,6 +1644,7 @@ nvc0_blitter_destroy(struct nvc0_screen *screen)
       }
    }
 
+   pipe_mutex_destroy(blitter->mutex);
    FREE(blitter);
 }