drm/amd/display: Hardcode dtbclk value in bw_params

drm/amd/display: Hardcode dtbclk value in bw_params

[why&how]

dtbclk should always be 600MHz. Previous logic was to get the real value
from SMU, but this returns 0 when dtbclk is off. Not a problem during
boot when pre-OS enables dtbclk, but PnP was broken due to this.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Matthew Stewart <Matthew.Stewart2@amd.com>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>