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 <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_irq_kms.c

@@ -145,7 +145,13 @@ void radeon_driver_irq_preinstall_kms(struct drm_device *dev)
  */
 int radeon_driver_irq_postinstall_kms(struct drm_device *dev)
 {
+	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;
 }