drm/amd/display: fix dppclk rcg poweron check

drm/amd/display: fix dppclk rcg poweron check

[WHY & HOW]
dppclk rcg power down will flip the poweron flag in the cache to cause dppclk rcg will never
run the rcg ungate sequence in some condition. Wait 10us to let dpp dto fully ramp.

Reviewed-by: Ovidiu (Ovi) Bunea <ovidiu.bunea@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Yihan Zhu <Yihan.Zhu@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>