drm/i915: FBC is updated within set_base() so remove second call in mode_set()

The FBC is dependent upon a few details of the framebuffer so it is
required to be updated within set_base(), so remove the redundant call
from mode_set().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>