drm/amdkfd: Add proper prefix to functions

Add amdgpu_amdkfd_ prefix to amdgpu functions served for amdkfd usage.

v2: fix indentation

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>