drm/i915: print FBC compression status on debugfs

We already had a few bugs in the past where FBC was compressing
nothing when it was enabled, which makes the feature quite useless.
Add this information to debugfs so the test suites can check for
regressions in this piece of the code.

Our igt/tests/kms_frontbuffer_tracking already has support for this
message.

v2: - Remove pointless VLV check (Ville).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>