drm/v3d: Checking for NULL vs IS_ERR()

The v3d_fence_create() only returns error pointers on error.  It never
returns NULL.

Fixes: 57692c94

 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518081041.GC28335@mwanda