drm/amdgpu/navi10: fix unsigned comparison with 0

Fixes warning because size is uint32_t and can never be negtative

drivers/gpu/drm/amd/amdgpu/../powerplay/navi10_ppt.c:1296:12: warning:
comparison of unsigned expression < 0 is always false [-Wtype-limits]
   if (size < 0)

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: ChenTao <chentao107@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>