drm/i915: simplify struct drm_device access at intel_atomic_check()

We already have a dev variable, there's no need to access state->dev.
Also, I plan to add another dev_priv user here, so declare one for the
current user.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-17-git-send-email-paulo.r.zanoni@intel.com