drm/i915: pass which operation triggered the frontbuffer tracking

We want to port FBC to the frontbuffer tracking infrastructure, but
for that we need to know what caused the object invalidation so
we can react accordingly: CPU mmaps need manual, GTT mmaps and
flips don't need handling and ring rendering needs nukes.

v2: - s/ORIGIN_RENDER/ORIGIN_CS/ (Daniel, Rodrigo)
    - Fix copy/pasted wrong documentation
    - Rebase
v3: - Rebase
v4: - Don't pass the operation to flushes (Daniel).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>