drm/amd/display: Fix unsigned comparison to zero

Fixes coccicheck warning:

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c:1398:60-61:
WARNING: Unsigned expression compared with zero: j >= 0

Fixes: 23838777 ("drm/amd/display: fix rn soc bb update")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>