drm/amdgpu: Using uninitialized value *size when calling amdgpu_vce_cs_reloc

Initialize the size before calling amdgpu_vce_cs_reloc, such as case 0x03000001.
V2: To really improve the handling we would actually
   need to have a separate value of 0xffffffff.(Christian)

Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>