drm/amd/display: fix wrong index used in dccg32_set_dpstreamclk

[Why & How]
When merging commit 9af611f2
("drm/amd/display: Fix DCN32 DPSTREAMCLK_CNTL programming"),
index change was not picked up.

Cc: stable@vger.kernel.org
Cc: Mario Limonciello <mario.limonciello@amd.com>
Fixes: 9af611f2 ("drm/amd/display: Fix DCN32 DPSTREAMCLK_CNTL programming")
Reviewed-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>