drm/i915: Avoid computing tile_row_size() for untiled objects

i915_gem_tile_height() asserts that the object is tiled, but inside the
error printer for the selftest we computed the row size regardless of
tiling, tripping over the assert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180726104759.8684-1-chris@chris-wilson.co.uk