drm/amd/display: Merge pipes for validate

drm/amd/display: Merge pipes for validate

Validation expects to operate on non-split pipes. This is
seen in dcn20_fast_validate_bw, which merges pipes for
validation. We weren't doing that in the non-fast path
which lead to validation failures when operating with
4-to-1 MPC and a writeback connector.

Co-developed by Claude Sonnet 4.5

Assisted-by: Claude:claude-sonnet-4.5
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>