drm/amd/display: ASSERT when failing to find index by plane/stream id

[WHY]
find_disp_cfg_idx_by_plane_id and find_disp_cfg_idx_by_stream_id returns
an array index and they return -1 when not found; however, -1 is not a
valid index number.

[HOW]
When this happens, call ASSERT(), and return a positive number (which is
fewer than callers' array size) instead.

This fixes 4 OVERRUN and 2 NEGATIVE_RETURNS issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>