drm/i915: reduce some of the duplication of tiling checking

i915_gem_object_fenceable was mostly just a repeat of the
i915_gem_object_fence_offset_ok, but also checking the size (which was
checkecd when we allowed that BO to be tiled in the first place). So
instead, export the latter function and use it in place.

Signed-Off-By: Owain G. Ainsworth <oga@openbsd.org>
Signed-off-by: Eric Anholt <eric@anholt.net>