drm/amd/display: do not force UCLK DPM to stay at highest state during display off in DCN2

[Why]
Add optimization to allow pstate change support when all displays
are off in DCN2.

[How]
Add clk_mgr_helper_get_active_plane_cnt() to sum plane_count for all
valid stream_status[].  If plane_count is 0, then there are no active
or virtual streams present. Use plane_count == 0 as extra condition to
enable p_state_change_support in dcn2_update_clocks().

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>