drm/amdgpu: retire unused get_umc_v6_7_channel_index

Fix the following compile warning:

drivers/gpu/drm/amd/amdgpu/umc_v6_7.c:53:24: warning: unused function 'get_umc_v6_7_channel_index' [-Wunused-function]
static inline uint32_t get_umc_v6_7_channel_index(struct amdgpu_device *adev,
                          ^
1 warning generated.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>