amdgpu/: concisely && consistently check null ptrs in canonical form

Be consistent and use the canonical form while sanity checking
null pointers, also combine a few branches for brevity.

v2: rebase on top of 'add amdgpu_cs_wait_fences' series.

Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>