xd86drm: read more than 128 bytes of uevent in drmParsePciBusInfo

Some platforms (such as Macs using OF) can have more information in the
uevent file thus reading only the first 128 might not be sufficient.

Bump it to 512, which "should be enough for everybody" ;-)

v2: Use sizeof(data)-1 over hardcoded number (Eric).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98629


Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reported-by: Mingcong Bai <jeffbai@aosc.xyz>
Tested-by: Mingcong Bai <jeffbai@aosc.xyz> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>