drm/i915/migrate: fix length calculation

drm/i915/migrate: fix length calculation

[ Upstream commit 31d70749bfe110593fbe8bf45e7c7788c7d85035 ]

No need to insert PTEs for the PTE window itself, also foreach expects a
length not an end offset, which could be gigantic here with a second
engine.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206112539.3149779-3-matthew.auld@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>