drm/i915: Use dev_priv consistently through the intel_frontbuffer interface

Rather than a mismash of struct drm_device *dev and struct
drm_i915_private *dev_priv being used freely within a function, be
consistent and only pass along dev_priv.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-22-git-send-email-chris@chris-wilson.co.uk