drm/i915: Process page flags once rather than per pwrite/pread

We used to lock individual pages inside the buffer object and so needed
to update the page flags every time. However, we now pin the pages into
the object for the duration of the pwrite/pread (and hopefully much
longer) and so we can forgo the flag updates until we release all the
pages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>