drm/amdgpu: remove unused functions

drm/amdgpu: remove unused functions

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.c:146:5: warning: no previous prototype for 'pool_to_domain' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/cz_smc.c:104:5: warning: no previous prototype for 'cz_send_msg_to_smc_with_parameter_async' [-Wmissing-prototypes]

In fact, both functions are called by no one and not exported,
so this patch removes them.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>