drm/hisilicon/hibmc: Checking for NULL instead of IS_ERR()

The drm_dev_alloc() function returns error pointers.  It never returns
NULLs.

Fixes: 5e0df3a0 ("drm/hisilicon/hibmc: Add hisilicon hibmc drm master driver")
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/20161124113545.GP17225@mwanda