drm/ttm: check the return value of kzalloc

In the function ttm_page_alloc_init, kzalloc call is made for variable
_manager, we need to check its return value, it may return NULL.

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>