drm/amdgpu: fix unnecessary NULL check warnings

Remove NULL checks before vfree() to fix these warnings:
drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c:102:2-8: WARNING: NULL
check before some freeing functions is not needed.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>