drm/amdgpu: Fix-ups for amdgpu_object.c documentation

* Fix format of return value descriptions
* Document all parameters of amdgpu_bo_free_kernel
* Document amdgpu_bo_get_preferred_pin_domain

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>