drm/amd/display: Allow bandwidth validation for 0 streams.

[Why]
Bandwidth calculations are triggered for non zero streams, and
in case of 0 streams, these calculations were skipped with
pstate status not being updated.

[How]
As the pstate status is applicable for non zero streams, check
added for allowing 0 streams inline with dcn internal bandwidth
validations.

Signed-off-by: Bindu Ramamurthy <bindu.r@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>