local-display-factory: look for 'boot_display' sysfs attr

ceromarequested to merge
ceroma/gdm:drm_boot_display into main

Some modern GPUs (like AMD's Strix Halo/Point) don't support VGA, but GDM currently looks for the 'boot_vga' flag before it creates a display.

Latest kernels introduced a 'boot_display' attribute (https://lkml.org/lkml/2025/7/17/1273) that can be used to detect which device was the primary display.

This diff updates the local display creation to search for the new flag in addition to the old VGA one.

Edited by ceroma

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