drm/i915: add HAS_DDI check

And use it whenever we call code that uses the DDIs. We already have
intel_ddi.c and prefix every function with intel_ddi_something instead of
haswell_something, so I think replacing the checks with HAS_DDI makes more
sense. Just a cosmetical change, yes I know, but I have this OCD...

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 9be7efd..e908c49 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1208,6 +1208,8 @@
 
 #define HAS_PIPE_CONTROL(dev) (INTEL_INFO(dev)->gen >= 5)
 
+#define HAS_DDI(dev)		(IS_HASWELL(dev))
+
 #define INTEL_PCH_DEVICE_ID_MASK		0xff00
 #define INTEL_PCH_IBX_DEVICE_ID_TYPE		0x3b00
 #define INTEL_PCH_CPT_DEVICE_ID_TYPE		0x1c00