drm/i915: Fix potential overflows in ilk ips calculations

A bunch of the ips calculations require 64bit math. In particular
'corr' and 'corr2' look like they can overflow on 32bit systems.
Switch to explicit u64 for those.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201021131443.25616-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>