drm/amd/display: Use tg count for opp init.

In case of tg count not equal to FE pipe count, if use pipe count to iterate
the tgs, it will cause BSOD.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>