drm/i915: drop the typedef for drm_i915_private_t

There are no longer users of drm_i915_private_t. Drop the typedef. Good
riddance.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Chris Wilson <chris@chris-wislon.co.uk>
[danvet: Add the hunk in i915_cmd_parser.c here which had to be
relocated to the how this was merged.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>