lib/igt_pm: make igt_pm_dmc_loaded() more robust

Make igt_pm_dmc_loaded() more robust against changes in the debugfs
file. Read the entire contents of the file, at least with the current
output. Preserve logged output even if something gets added in front of
the file, and don't depend on exact length of the first line.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>