drm/amdgpu: fix amdgpu_preempt_mgr_new()

There is a reversed if statement in amdgpu_preempt_mgr_new() so it
always returns -ENOMEM.

Fixes: 09b020bb05a5 ("Merge tag 'drm-misc-next-2021-06-09' of git://anongit.freedesktop.org/drm/drm-misc into drm-next")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/YMxbQXg/Wqm0ACxt@mwanda
Signed-off-by: Christian König <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_preempt_mgr.c

···
 	struct amdgpu_preempt_mgr *mgr = to_preempt_mgr(man);
 
 	*res = kzalloc(sizeof(**res), GFP_KERNEL);
-	if (*res)
+	if (!*res)
 		return -ENOMEM;
 
 	ttm_resource_init(tbo, place, *res);