drm/i915: also do frontbuffer tracking on pwrites

We need this for FBC, and possibly for PSR too.

v2: Don't only flush: invalidate too (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>