drm/amd/amdgpu: Add offset variant to SOC15 macros

Allows reading/writing via SOC15 macros with offset for
various register banks.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>