drm/amdgpu: add macro to calculate offset with instance

Add macro definition which calculate offset of the
register with index override.

This is useful in case when there is an array of
registers which is common for all instances.
To read registers in that case it is easy to define
registers once and the index value is manually passed
to calculate proper offset of register for each instance.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>