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>

drivers/gpu/drm/amd/display/dc/dml2/dml2_dc_resource_mgmt.c

@@ -88,7 +88,8 @@ static int find_disp_cfg_idx_by_plane_id(struct dml2_dml_to_dc_pipe_mapping *map
 			return  i;
 	}
 
-	return -1;
+	ASSERT(false);
+	return __DML2_WRAPPER_MAX_STREAMS_PLANES__;
 }
 
 static int find_disp_cfg_idx_by_stream_id(struct dml2_dml_to_dc_pipe_mapping *mapping, unsigned int stream_id)
@@ -100,7 +101,8 @@ static int find_disp_cfg_idx_by_stream_id(struct dml2_dml_to_dc_pipe_mapping *ma
 			return  i;
 	}
 
-	return -1;
+	ASSERT(false);
+	return __DML2_WRAPPER_MAX_STREAMS_PLANES__;
 }
 
 // The master pipe of a stream is defined as the top pipe in odm slice 0