drm/amdgpu: Use struct_size() helper in kmalloc()

Make use of the new struct_size() helper instead of the offsetof() idiom.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>