xfree86/modes: Use per-screen monitor for all outputs

Instead of just the first one.

This allows 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).

Had the idea of this change after noticing that an actual Monitor section with
DisplaySize was not taken, because indeed the monitor was not connected to the
first output.

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 see 2.).
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 (to be checked by someone more
experienced).

At least 'xf86-video-intel' may need to be updated accordingly (see
'src/sna/sna_display.c', function 'sna_output_add', call to
'xf86OutputUseScreenMonitor'). Didn't check drivers besides 'xf86-video-intel',
'xf86-video-amdgpu', 'xf86-video-nouveau', 'xf86-video-nv' and
'xf86-video-vesa'.

Signed-off-by: Olivier Certner <olce.freedesktop@certner.fr>
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