drm/amd/display: Reset pipe count when iterating for DET override

[Why]
Coding error in DET allocation was resulting in too few DET segments
being allocated, causing underflow.

[How]
Reset pipe count each time we begin iterating through pipes for a stream.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Taimur Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>