drm/amd/display: Fix update_bw_bounding_box Calcs

[Why]
Previously update_bw_bounding_box for RN was commented out
due to incorrect values causing BSOD on Hybrid Graphics.
However, commenting out this function also may cause issues
such as underflow in certain cases such as 2x4K displays.

[How]
Fix dram_speed_mts calculations.
Update from proper index of clock_limits[]

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>