drm/i915/gtt: Reduce a pair of runtime asserts

We can stop asserting using WARN_ON as given sufficient CI coverage, we
can rely on using GEM_BUG_ON() to catch problems before merging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614184218.1606-2-chris@chris-wilson.co.uk