drm/amdgpu/soc15: drop support for reading some registers

The RB harvest registers are not necessary, the driver already
exposes this info via the info ioctl.  GB_BACKEND_MAP has
been deprecated since SI and is not relevant to the RB mapping.

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