drm/i915: Mass convert dev->dev_private to to_i915(dev)

Since we now subclass struct drm_device, we can save pointer dances by
noting the equivalence of struct drm_device and struct drm_i915_private,
i.e. by using to_i915().

   text    data     bss     dec     hex filename
1073824    4562     416 1078802  107612 drivers/gpu/drm/i915/i915.ko
1068976    4562     416 1073954  106322 drivers/gpu/drm/i915/i915.ko

Created by the coccinelle script:

@@
expression E;
identifier p;
@@
- struct drm_i915_private *p = E->dev_private;
+ struct drm_i915_private *p = to_i915(E);

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467628477-25379-1-git-send-email-chris@chris-wilson.co.uk