drm/i915: fix bpc vs. bpp confusion in intel_crtc_compute_config

Oops.

This regression has been introduced in

commit 5d2d38dd
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 27 00:45:01 2013 +0100

    drm/i915: clean up pipe bpp confusion

Reported-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>