drm/amd/display: fix counter in wait_for_no_pipes_pending

[Why]
Wait counter is not being reset for each pipe.

[How]
Move counter reset into pipe loop scope.

Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>