drm/i915: use dev_priv for the FBC functions

Because the cool kids use dev_priv and FBC wants to be cool too.

We've been historically using struct drm_device on the FBC function
arguments, but we only really need it for intel_vgpu_active(): we can
use dev_priv everywhere else. So let's fully switch to dev_priv since
I'm getting tired of adding "struct drm_device *dev = dev_priv->dev"
everywhere.

If I get a NACK here I'll propose the opposite: convert all the
functions that currently take dev_priv to take dev.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>