drm/amdgpu: Set vblank_disable_allowed = true

Without this, since the conversion from drm_vblank_pre/post_modeset to
drm_vblank_on/off, the vblank interrupt could never be disabled after
userspace triggered enabling it.
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -219,6 +219,8 @@ int amdgpu_irq_init(struct amdgpu_device *adev)
 	if (r) {
 		return r;
 	}
+	adev->ddev->vblank_disable_allowed = true;
+
 	/* enable msi */
 	adev->irq.msi_enabled = false;