drm/i915: Introduce i915_vgacntrl_reg()

The VGACNTRL register has moved around between different platforms.
To handle the differences add i915_vgacntrl_reg() which returns the
correct offset for the VGACNTRL register.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -1887,4 +1887,14 @@ __i915_write(64, q)
 #define INTEL_BROADCAST_RGB_FULL 1
 #define INTEL_BROADCAST_RGB_LIMITED 2
 
+static inline uint32_t i915_vgacntrl_reg(struct drm_device *dev)
+{
+	if (HAS_PCH_SPLIT(dev))
+		return CPU_VGACNTRL;
+	else if (IS_VALLEYVIEW(dev))
+		return VLV_VGACNTRL;
+	else
+		return VGACNTRL;
+}
+
 #endif