drm/amd/display: fix array-bounds errors in dc_stream_remove_writeback()

The following errors occurred when using gcc 7.5.0-3ubuntu1~18.04:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c: In function ‘dc_stream_remove_writeback’:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:543:55: warning: array subscript is above array bounds [-Warray-bounds]
     stream->writeback_info[j] = stream->writeback_info[i];
                                 ~~~~~~~~~~~~~~~~~~~~~~^~~
Add a check to make sure that num_wb_info won't overflowing the writeback_info buffer.

Fixes: 6fbefb84 ("drm/amd/display: Add DC core changes for DCN2")
Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_stream.c

@@ -481,6 +481,7 @@ bool dc_stream_add_writeback(struct dc *dc,
 	}
 
 	if (!isDrc) {
+		ASSERT(stream->num_wb_info + 1 <= MAX_DWB_PIPES);
 		stream->writeback_info[stream->num_wb_info++] = *wb_info;
 	}
 
@@ -526,6 +527,11 @@ bool dc_stream_remove_writeback(struct dc *dc,
 		return false;
 	}
 
+	if (stream->num_wb_info > MAX_DWB_PIPES) {
+		dm_error("DC: num_wb_info is invalid!\n");
+		return false;
+	}
+
 //	stream->writeback_info[dwb_pipe_inst].wb_enabled = false;
 	for (i = 0; i < stream->num_wb_info; i++) {
 		/*dynamic update*/
@@ -540,7 +546,8 @@ bool dc_stream_remove_writeback(struct dc *dc,
 		if (stream->writeback_info[i].wb_enabled) {
 			if (j < i)
 				/* trim the array */
-				stream->writeback_info[j] = stream->writeback_info[i];
+				memcpy(&stream->writeback_info[j], &stream->writeback_info[i],
+						sizeof(struct dc_writeback_info));
 			j++;
 		}
 	}