drm/i915: tame the chattermouth (v2)

Many distro's have mechanism in place to collect and automatically file
bugs for failed WARN()s.  And since i915 has a lot of hw state sanity
checks which result in WARN(), it generates quite a lot of noise which
is somewhat disconcerting to the end user.

Separate out the internal hw-is-in-the-state-I-expected checks into
I915_STATE_WARN()s and allow configuration via i915.verbose_checks module
param about whether this will generate a full blown stacktrace or just
DRM_ERROR().  The new moduleparam defaults to true, so by default there
is no change in behavior.  And even when disabled, you will still get
an error message logged.

v2: paint the macro names blue, clarify that the default behavior
    remains the same as before

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>