drm/amdgpu: Remove unused get_umc_v8_7_channel_index function

Remove get_umc_v8_7_channel_index function, which is not used
in the codebase.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/umc_v8_7.c:50:24: warning: unused function
'get_umc_v8_7_channel_index' [-Wunused-function]
static inline uint32_t get_umc_v8_7_channel_index(struct amdgpu_device *adev,
                       ^

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>