drm/i915: prefer struct drm_i915_private to drm_i915_private_t

Remove the rest of the references to drm_i915_private_t. No functional
changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Drop hunk in i915_cmd_parser.c]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>