drm/i915: kick out cmd_parser specific structs from i915_drv.h

No sense in keeping the cmd_descriptor and cmd_table structs in
i915_drv.h, now that they are no longer referenced externally.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479942147-9837-1-git-send-email-matthew.auld@intel.com