drm/i915: remove unused IO_TLB_SEGPAGES which should be defined by swiotlb

This patch removes IO_TLB_SEGPAGES which is no longer used since
commit 5584f1b1

 ("drm/i915: fix i915 running as dom0 under Xen").

As the define of both IO_TLB_SEGSIZE and IO_TLB_SHIFT are from swiotlb,
IO_TLB_SEGPAGES should be defined on swiotlb side if it is required in the
future.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1558413639-22568-1-git-send-email-dongli.zhang@oracle.com