drm/i915: Use an intermediate variable to avoid repeating ourselves

The code look slightly better this way and will ease the next commit,
changing where we take the fb pointer from.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>