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

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

This reverts commit 406bf325f00152e514086ee3550d03e108b3caa0.

The same warning has been fixed in 24df789b739ffd88585892913e015c649f14646f 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>