drm/amdgpu: Address member 'gart_placement' not described in 'amdgpu_gmc_gart_location'

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c:274: warning: Function parameter or member 'gart_placement' not described in 'amdgpu_gmc_gart_location'

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>