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

In the Linux kernel, the following vulnerability has been resolved: 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.