drm/amd/display: Do not return negative stream id for array

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

[HOW]
When this happens, call ASSERT(), and return a zero instead.

This fixes an OVERRUN and an 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>