hw/xfree86/modes: Use per-screen monitor for all card outputs by default

Instead of just the first one.

An explicit DisplaySize directive in a Monitor section referenced by a Screen section in xorg.conf is not taken into account when the monitor is not connected to the Screen's card's first output.

Desired outcome: Apply the directives of the monitor section to all card's outputs that do not have an explicit monitor linked to them. This would allow to specify a monitor section for the screen without bothering to which actual card output the monitor is connected to (in a single-monitor setup).

Rationale:

1. Desktop/server graphic cards commonly have several outputs, even for a single output type (DP, HDMI, etc.). This would ease configuring for single-monitor cases. Multi-monitor setups require per-output monitor sections anyway (or autoconfiguration, see next point).
2. The change preserves compatibility with single-output setups and autoconfiguration.
3. Seems a priori that applying the per-screen monitor to the first output is rather arbitrary.