drm/etnaviv: add sensitive state for PE_RT_ADDR_4_PIPE(3, 0|1) address

0x1540 is the address of 4th render target address pair (two pixel pipes).

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>