drm/i915: Replace WARNs in fence register writes with extensive asserts

All of these conditions are prechecked by i915_tiling_ok() before we
allow setting the tiling/stride on the object and so we should never
fail asserting those conditions before writing the register.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109161613.11881-3-chris@chris-wilson.co.uk