lib/igt_kms.c: remove tests dependency on VT /dev/tty0
lib/igt_kms.c: remove tests dependency on VT /dev/tty0

Required to run on any recent, freon-based and X11-free ChromeOS release.

v2:
- igt_debug() instead of igt_warn()
- return KD_GRAPHICS instead of -1UL
- print previous mode in debug statements. Among others this help a tiny
  bit with the now confusing debug output ("cannot change" immediately
  followed by a misleading "mode changed").

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

chenxianqin/intel-gpu-tools,mv0/intel-gpu-tools,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,dlespiau/checkmate-test-igt,tiagovignatti/intel-gpu-tools,dlespiau/checkmate-test-igt,chenxianqin/intel-gpu-tools,tiagovignatti/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools