drm/amd/display: Clear the CUR_ENABLE register on DCN20 on DPP5

drm/amd/display: Clear the CUR_ENABLE register on DCN20 on DPP5

[Why]
On DCN20 & DCN30, the 6th DPP's & HUBP's are powered on permanently and
cannot be power gated. Thus, when dpp_reset() is invoked for the DPP5,
while it's still powered on, the cached cursor_state
(dpp_base->pos.cur0_ctl.bits.cur0_enable)
and the actual state (CUR0_ENABLE) bit are unsycned. This can cause a
double cursor in full screen with non-native scaling.

[How]
Force disable cursor on DPP5 on plane powerdown for ASICs w/ 6 DPPs/HUBPs.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4673
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>