vk: Move gen8 specific state into gen8 sub-structs

vk: Move gen8 specific state into gen8 sub-structs

This commit moves all occurances of gen8 specific state into a gen8
substruct. This clearly identifies the state as gen8 specific and
prepares for adding gen7 state structs. In the process we also rename
the field names to exactly match the command or state packet name,
without the 3DSTATE prefix, eg:

  3DSTATE_VF -> gen8.vf
  3DSTATE_WM_DEPTH_STENCIL -> gen8.wm_depth_stencil

Signed-off-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>