drm/i915: Remove vestigial pitch from post-gen2 FBC control routines

The cfb_pitch was only used for 8xx_enable_fbc(), every later routine
was just overwriting the value with itself thanks to a copy'n'paste
error.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>