drm/i915: Performed deferred clflush inside set-cache-level

Currently we are hitting the WARN inside
i915_gem_object_set_cache_level() as we can now have an unbound object
in the GTT write domain (due to 43566ded "drm/i915: Broaden
application of set-domain(GTT)"). To avoid the warning, we need to track
when we elided the clflush on a cacheable object and then evict the
cache for the object when we move the object out of a cacheable domain.

Reported-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Testcase: igt/gem_mmap_wc/set-cache-level
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88607


Tested-by:  <huax.lu@intel.com>
[danvet: Split if into nested if as discussion on the m-l.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>