drm/i915: add null render states for gen6, gen7 and gen8

These are generated with intel-gpu-tools/tools/null_state_gen

v2: Don't use header file for states (Daniel Vetter)

v3: Proper URB state size for gen8/GT3 (Damien Lespiau)

Tested-by: Kristen Carlson Accardi <kristen@linux.intel.com> (v1)
Tested-by: Oscar Mateo <oscar.mateo@intel.com> (v2)
Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>