drm/i915: fix up a raw 64bit divide

We are seeing compilation failures on i386 in some environments due
to an undefined reference as below:

    ERROR: "__udivdi3" [drivers/gpu/drm/i915/i915.ko] undefined!

This is generated due to a raw 64 bit divide in the i915 driver.  Fix up
this raw divide.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>