drm/amdgpu: Fix logic to fetch supported NPS modes

Correct the logic to find supported NPS modes from firmware.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reported-by: Ava Zhang <niandong.zhang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Fixes: 30eb41f5 ("drm/amdgpu: Use firmware supported NPS modes")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>