drm/amdgpu: remove grbm cam index/data operations for gfx v10

PSP firmware will be responsible for applying the GRBM CAM remapping in
the production. And the GRBM_CAM_INDEX / GRBM_CAM_DATA registers will be
protected by PSP under security policy. So remove it according to the
new security policy.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>