drm/amd/display: Fix Vega10 black screen after mode change

drm/amd/display: Fix Vega10 black screen after mode change

[Why]
The sequence is slightly changed when bring .set_bandwidth out
from the end of programming backend to the end of programming
surface. Vega10 doesn't like to get clocks updated if
stream_count is zero in the current context (Atomic Reset).

[How]
Do not update clocks if no stream is showing up in the context.

Fixes 1b2b130192 "dc: Remove 300Mhz minimum disp clk limit."

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@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>