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>

drivers/gpu/drm/i915/intel_lrc.c

@@ -1319,7 +1319,7 @@ static int gen8_emit_request(struct intel_ringbuffer *ringbuf)
 	if (ret)
 		return ret;
 
-	cmd = MI_STORE_DWORD_IMM_GEN8;
+	cmd = MI_STORE_DWORD_IMM_GEN4;
 	cmd |= MI_GLOBAL_GTT;
 
 	intel_logical_ring_emit(ringbuf, cmd);