drm/amdgpu: Remove amdgpu_gem_map_attach target_dev documentation

The parameter was removed.

Fixes: a19741e5 "dma_buf: remove device parameter from attach
                     callback v2"
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>