drm/amd/display: DCHUB requestors numbers for Navi.

[Why]
The DCHub arbiter has a mechanism to dynamically rate limit the DCHub
request stream to the fabric. If the memory controller is fully utilized
and the DCHub requestors are well ahead of their amortized schedule,
then it is safe to prevent the next winner from being committed and sent
to the fabric. The utilization of the memory controller is approximated
by ensuring that the number of outstanding requests is greater than a
threshold specified by the ARB_MIN_REQ_OUTSTANDING. To determine that
the DCHub requestors are well ahead of the amortized schedule, the slack
of the next winner is compared with the ARB_SAT_LEVEL in DLG RefClk
cycles.

[How]
The recommended settings to enable thise dynamic limiting for Navi is
180 requests.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>