drm/i915: debugfs for frontbuffer tracking

Useful to figure out whether stuck bits are due to the frontbuffer
tracking code as opposed to individual consumers (who have their own
bitmask tracking).

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

diff --cc drivers/gpu/drm/i915/i915_debugfs.c
Simple merge