Don't call scanout_flip/update with a legacy RandR scanout buffer

It means we are not using our own scanout buffers.

Fixes crash when TearFree is supposed to be enabled, but
drmmode_handle_transform doesn't set crtc->driverIsPerformingTransform.

Bugzilla: https://bugs.freedesktop.org/105736


(Ported from amdgpu commit 463477661c88cab3a87746499e5838c5b9f9a13b)
Acked-by: Alex Deucher <alexander.deucher@amd.com>