drm/i915: Amalgamate flushing of display objects

We have three different paths by which userspace wants to flush the
display plane (i.e. objects with obj->pin_display). Use a common helper
to identify those paths and to simplify a later change.

v2: Include the conditional in the name, i915_gem_object_flush_if_display

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170222114049.28456-3-chris@chris-wilson.co.uk