drm/amdgpu/gmc: move invaliation bitmap setup to common code

So it can be shared with newer GMC versions.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>