drm/i915: s/MI_STORE_DWORD_IMM_GEN8/MI_STORE_DWORD_IMM_GEN4/

MI_STORE_DWORD_IMM length has been the same ever since gen4. Rename
the define to avoid potential confusion if someone tries to use this
on pre-gen8.

Also correct the comment on MI_MEM_VIRTUAL bit. It's present on 945,g33
and 965 only.

Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add USE_GGTT define for g4x+ too.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>