drm: Unconditionally do the range check in drm_mm_scan_add_block()

Doing the check is trivial (low cost in comparison to overall eviction)
and helps simplify the code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-29-chris@chris-wilson.co.uk