lib/drmtest: fix hang in quiescent_gpu_at_exit due to signal-unsafe asprintf

lib/drmtest: fix hang in quiescent_gpu_at_exit due to signal-unsafe asprintf

I got kms_flip in hung state with the backtrace below, while the parent process
waiting for the signal helper to exit. It was quite easy to reproduce the bug
by running

kms_flip --run-subtest=flip-vs-dpms-off-vs-modeset

With the change I couldn't reproduce it.

TODO: audit/fix other signal handlers with signal-unsafe functions

0  0x00007f9a1362018b in ?? () from /lib/x86_64-linux-gnu/libc.so.6
1  0x00007f9a1359df81 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
2  0x00007f9a1359b6cf in ?? () from /lib/x86_64-linux-gnu/libc.so.6
3  0x00007f9a13628eb6 in __vasprintf_chk () from /lib/x86_64-linux-gnu/libc.so.6
4  0x00007f9a13628e72 in __asprintf_chk () from /lib/x86_64-linux-gnu/libc.so.6
5  0x000000000040a4a2 in asprintf (__fmt=0x417441 "/dev/dri/card%u", __ptr=0x7fff1a972c08)
    at /usr/include/x86_64-linux-gnu/bits/stdio2.h:178
6  drm_get_card () at drmtest.c:190
7  0x000000000040a54a in __drm_open_any () at drmtest.c:229
8  0x000000000040a846 in quiescent_gpu_at_exit (sig=<optimized out>) at drmtest.c:281
9  0x0000000000408759 in call_exit_handlers (sig=3) at drmtest.c:1519
10 fatal_sig_handler (sig=3) at drmtest.c:1543
11 <signal handler called>
12 0x00007f9a13596770 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
13 0x00007f9a135d8f3f in fork () from /lib/x86_64-linux-gnu/libc.so.6
14 0x000000000040b3af in __igt_fork_helper (proc=0x61d8cc <signal_helper>) at drmtest.c:1199
15 0x000000000040b4ce in igt_fork_signal_helper () at drmtest.c:751
16 0x0000000000404167 in main (argc=<optimized out>, argv=<optimized out>) at kms_flip.c:1533

v2:
- can't use snprintf as that's also signal-unsafe, so just use a dup'ed fd (Chris,Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>