drm, kdb, kms: Change mode_set_base_atomic() enter argument to be an enum

The enter argument as implemented by commit 413d45d3627 (drm, kdb, kms:
Add an enter argument to mode_set_base_atomic() API) should be more
descriptive as to what it does vs just passing 1 and 0 around.

There is no runtime behavior change as a result of this patch.

Reported-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: David Airlie <airlied@linux.ie>
CC: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>