drm/i915/userptr: add user_size limit check

Don't allow a mismatch between obj->base.size/vma->size and the actual
number of pages for the backing store, which is limited to INT_MAX
pages.

v2: document what are missing before we can safely drop the limit check

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117132413.1170563-1-matthew.auld@intel.com