backends/drm: only enforce modifiers default for surface creation

Whether or not we want to use explicit modifiers for our surfaces doesn't matter for what format+modifiers drm planes support. This way direct scanout works by default, without having to explicitly enable modifiers.

In combination with !1706 (merged), mesa!11248 and the Vulkan equivalent for mesa!11248 we should finally pretty much always have direct scanout when in fullscreen. Only thing still missing is scaling