drm/amd/amdgpu: gfx9 tidy ups (v2)

A couple of simple tidy ups to register programming.

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

(v2): Avoid using 'data' uninitialized

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>