v3d: Fix a leak of the transfer helper on screen destroy.

Fixes: 7a30517c ("broadcom/vc5: Start adding support for rendering to Z32F_S8X24_UINT.")
(cherry picked from commit 3bd73d31)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/gallium/drivers/v3d/v3d_screen.c