drm/amdgpu: remove unused definition

mmhub_v1_8_mmea_cgtt_clk_cntl_reg is defined but not used.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Acked-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>