drm/amdkfd: Check for null pointer after calling kmemdup

As the possible failure of the allocation, kmemdup() may return NULL
pointer.
Therefore, it should be better to check the 'props2' in order to prevent
the dereference of NULL pointer.

Fixes: 3a87177e ("drm/amdkfd: Add topology support for dGPUs")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>