drm/amd/amdkfd: adjust dummy functions' placement

Move all the dummy functions in amdgpu_amdkfd.c to
amdgpu_amdkfd.h as inline functions.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Suggested-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>