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

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: 6fbefb84a98e ("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>