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

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

This reverts commit 60f2b4af1258c05e6b037af866be81abc24438f7.

The same warning has been fixed in e5081a538a565284fec5f30a937d98e460d5e780 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>