drm/amd/display: Remove useless function call

[WHAT & HOW]
clk_mgr_helper_get_active_display_cnt returns display count and itself
alone has no use.

This fixes 1 CHECKED_RETURN issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>