vc4: Fix a leak of the transfer helper on screen destroy.

Fixes: d009463a ("vc4: Switch to using u_transfer_helper for MSAA maps.")
(cherry picked from commit 7f8d8b7d)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/gallium/drivers/vc4/vc4_screen.c