drm/i915/gem: Promote 'remain' to unsigned long

Tidy the code by casting remain to unsigned long once for the duration
of eb_relocate_vma()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200407085930.19421-1-chris@chris-wilson.co.uk