drm/amd/display: Program DWB watermarks from correct state

[Why]
When diags adds a DWB via a stream update, we calculate MMHUBBUB
paramaters, but dc->current_state has not yet been updated
when the DWB programming happens. This leads to overflow on
high bandwidth tests since the incorrect MMHUBBUB arbitration
parameters are programmed.

[How]
Pass the updated context down to the (enable|update)_writeback functions
so that they can use the correct watermarks when programming MMHUBBUB.

Signed-off-by: Julian Parkin <julian.parkin@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>