drm/i915: Make vblank evade warnings optional

Add a new Kconfig option to enable/disable the extra warnings
from the vblank evade code. For now we'll keep the warning
about an actually missed vblank always enabled as that can have
an actual user visible impact. But if we miss the deadline
othrwise there's no real need to bother the user with that.
We'll want these warnings enabled during development however
so that we can catch regressions.

Based on the reports it looks like this is still very easy
to hit on SKL, so we have more work ahead of us to optimize
the crtiical section further.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: Jens Axboe <axboe@kernel.dk>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: e1edbd44 ("drm/i915: Complain if we take too long under vblank evasion.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>