drm/amd/amdgpu: skip ucode loading if ucode_size == 0

[ Upstream commit 39c84b8e ]

Restrict the ucode loading check to avoid frontdoor loading error.

Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>