drm/i915: Add debugfs file to clear FIFO underruns.

Adding a i915_fifo_underrun_reset debugfs file will make it possible
for IGT tests to clear FIFO underrun fallout at the start of each
subtest, and make re-enable FBC so tests always have maximum exposure
to features used by IGT. FIFO underruns and FBC bugs will no longer
hide when an earlier subtests disables both.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=105685
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105681
Link: https://patchwork.freedesktop.org/patch/msgid/20180328100526.36467-1-maarten.lankhorst@linux.intel.com


Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
[mlankhorst: Reset FBC reason if underrun had occurred. (vivijim)]
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>