drm: mxsfb: drm_dev_alloc() returns error pointers

We should be checking for IS_ERR() instead of NULL because
drm_dev_alloc() returns error pointers.

Fixes: 45d59d70 ("drm: Add new driver for MXSFB controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213122332.GA7519@elgon.mountain