drm/amdgpu: 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.

Ported from radeon commit:
b0b9bb4dReviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/amdgpu/amdgpu_irq.c

@@ -140,7 +140,7 @@ void amdgpu_irq_preinstall(struct drm_device *dev)
  */
 int amdgpu_irq_postinstall(struct drm_device *dev)
 {
-	dev->max_vblank_count = 0x001fffff;
+	dev->max_vblank_count = 0x00ffffff;
 	return 0;
 }