drm/radeon: 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/radeon/radeon_irq_kms.c

@@ -291,6 +291,8 @@ int radeon_irq_kms_init(struct radeon_device *rdev)
 	if (r) {
 		return r;
 	}
+	rdev->ddev->vblank_disable_allowed = true;
+
 	/* enable msi */
 	rdev->msi_enabled = 0;