drm/i915: s/drm_i915_private_t/struct drm_i915_private/

People don't like typedefs these days. Eliminate their use from intel_fb.c.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>