drm/i915: Factor in pixel-repeat in FDI M/N calculation

Fixes the modesetting on the secondary panel of the Libretto W100 and
presumably many more Ironlake laptops with SDVO LVDS displays.

Reported-and-tested-by: Matthew Willoughby <mattfredwill@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org