Remove unused amdgpu_amdkfd_get_vram_usage()

CC: Felix.Kuehling@amd.com
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: dfcbe6d5 ("drm/amdgpu: Remove unused function pointers")