drm/nouveau/disp/gm200-: fix NV_PDISP_SOR_HDMI2_CTRL(n) selection

[ Upstream commit a1ef8bad ]

This is a SOR register, and not indexed by the bound head.

Fixes display not coming up on high-bandwidth HDMI displays under a
number of configurations.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>