drm/amd/display: early return when pipe_cnt is 0 in bw validation

[Why]
Unintentionally introduced behaviour change from previous refactor,
which causes clks to be 0 in no stream cases, which will cause
divide by 0.

[How]
Skip calculation of clocks when no stream. Which is the same as old
behaviour.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <tong.cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>