More fixes for modesetting for Server 21.1

Two commits, hopefully for inclusion into X-Server 21.1:

First one is an additional fix for VariableRefresh VRR mode, and also adds missing documentation for the VariableRefresh parameter, copied from amdgpu-ddx.

Second one adds the ability to use the designated "primary output" set via xrandr --output OUTPUT --primary as a tie-breaker if multiple outputs of a multi-display setup have the same maximal intersection area with a drawable and one output / crtc needs to be selected for synchronization and driving presentation timing and timestamping under OpenGL/Vulkan/Present. This is a straightforward port of the implementation in amdgpu-ddx and makes modesetting-ddx behaviour consistent with the behaviour of the vendor specific amdgpu-ddx, ati-ddx, nouveau-ddx. This is especially useful for mixed VRR + non-VRR display setups and when the new Option "AsyncFlipSecondaries" is used.