drm: Fix further issues in drivers/char/drm/via_irq.c

Fix de-reference of 'dev_priv' before NULL check.

Signed-off-by: Jayachandran C. <c.jayachandran@gmail.com>
Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@osdl.org>