drm/amd/display: Loop through all pipes for DET allocation

[Why & How]
There are cases where the pipes populated are not all at the top
of the pipes list under context. Loop through all pipes for DET
allocation instead of just the number of populated ones, even if
some unpopulated pipes are iterated through unnecessarily.

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