drm/amdgpu/gfx9: simplify and fix GRBM index selection

Copy the approach taken by gfx8, which simplifies the code, and set the
instance index properly. The latter is required for debugging, e.g. for
reading wave status by UMR.

Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>