drm/i915/fbc: disable FBC on FIFO underruns

Ever since I started working on FBC I was already aware that FBC can
really amplify the FIFO underrun symptoms. On systems where FIFO
underruns were harmless error messages, enabling FBC would cause the
underruns to give black screens.

We recently tried to enable FBC on Haswell and got reports of a system
that would hang after some hours of uptime, and the first bad commit
was the one that enabled FBC. We also observed that this system had
FIFO underrun error messages on its dmesg. Although we don't have any
evidence that fixing the underruns would solve the bug and make FBC
work properly on this machine, IMHO it's better if we minimize the
amount of possible problems by just giving up FBC whenever we detect
an underrun.

v2: New version, different implementation and commit message.
v3: Clarify the fact that we run from an IRQ handler (Chris).
v4: Also add the underrun_detected check at can_choose() to avoid
    misleading dmesg messages (DK).
v5: Fix Engrish, use READ_ONCE on the unlocked read (Chris).

Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Lyude <cpaul@redhat.com>
Cc: stevenhoneyman@gmail.com <stevenhoneyman@gmail.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473773937-19758-1-git-send-email-paulo.r.zanoni@intel.com