Revert "drm/i915: fix build warning on 32-bit (v2)"

Revert "drm/i915: fix build warning on 32-bit (v2)"

This reverts commit eaeacbc9dabb411342a807f66fe37e492e3a5997.

The same warning has been fixed in f0f8b97b82ba4a87608059b5567a544088892f26 and
these two commits got merged in 74e99a84de2d0980320612db8015ba606af42114 which
caused another warning. Simply, the reverted commit casted the pointer
difference to unsigned long and the other commit changed the output type from
long to ptrdiff_t.

The other commit fixes the original warning the better way so I'm reverting
this commit now.

Signed-off-by: Jan Moskyto Matejka <mq@suse.cz>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>