drm/i915: dev_priv cleanup in bridge/bar/mmio init code

dev_priv is more appropriate for these so converting saves
some lines of source.

v2: Commit message and keep the pdev local variable. (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>