xf86drm: fix null termination of string buffer

The string written to the buffer by read() is not null-terminated,
but currently drmParsePciBusInfo() places null character only at the end of the buffer, not at the end of the
string.
As a result, the string passed to sscanf() contains an uninitialized value.

This patch changes to places null character at the end of the string.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99045


Signed-off-by: Taro Yamada <archer_ame@yahoo.co.jp>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>