drm: Fix race when checking for fb in the generic kms obj lookup

In my review of

commit 98f75de4
Author: Rob Clark <robdclark@gmail.com>
Date:   Fri May 30 11:37:03 2014 -0400

    drm: add object property typ

I asked for a check to make sure that we never leak an fb from the
generic mode object lookup since those have completely different
lifetime rules. Rob added it, but outside of the idr mutex, which
means that our dereference of obj->type can already chase free'd
memory.

Somehow I didn't spot this, so fix this asap.

v2: Simplify the conditionals as suggested by Chris.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>