drm/radeon: Fix max_vblank_count value for current display engines

The value was much too low, which could cause the userspace visible
vblank counter to move backwards when the hardware counter wrapped
around.

Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Michel Dänzer <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon_irq_kms.c

···
  */
 int radeon_driver_irq_postinstall_kms(struct drm_device *dev)
 {
-	dev->max_vblank_count = 0x001fffff;
+	struct radeon_device *rdev = dev->dev_private;
+
+	if (ASIC_IS_AVIVO(rdev))
+		dev->max_vblank_count = 0x00ffffff;
+	else
+		dev->max_vblank_count = 0x001fffff;
+
 	return 0;
 }