drm/amd/amdgpu: Fix offset for vmid selection in debugfs interface

The register debugfs interface was using the wrong bitmask for vmid
selection for GFX_CNTL.

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