amd/amdgpu_ctx: Use struct_size() helper and kmalloc() (v2)

Make use of the new struct_size() helper instead of the offsetof() idiom.
Also, use kmalloc() instead of kcalloc().

v2: squash in kzalloc fix

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