drm/amd/display: fix pipe selection logic in validate

[why]
Resource mapping done in dcn20_validate_bandwidth has a flaw:  When a full
update is performed, the HWSS will only update the MPCC tree for the stream
that is updated as opposed to all streams.  This means that when mapping pipes
in validation, care must be taken to not change any existing mapping, otherwise it
leads to partial hw programming

[how]
it's not strictly necessary to track which stream/mpcc tree is being updated, but
rather it's sufficient to compare current and new state and just keep pipes that were
previously already mapped unchanged.

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>