drm/i915: don't clobber the pipe param in sanitize_modesetting

... we need it later on in the function to clean up pipe <-> plane
associations. This regression has been introduced in

commit f47166d2b0001fcb752b40c5a2d4db986dfbea68
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Mar 22 15:00:50 2012 +0000

    drm/i915: Sanitize BIOS debugging bits from PIPECONF

Spotted by staring at debug output of an (as it turns out) totally
unrelated bug.

v2: I've totally failed to do the s/pipe/i/ correctly, spotted by
Chris Wilson.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Cc: stable@kernel.org (the regression was Cc: stable, too)
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>