If we move the sanity checking from gen8_alloc_va_range_3lvl and
gen6_alloc_va_range into i915_vma_bind, we will increase our coverage to
now both callbacks. We also convert each WARN_ON over to a GEM_WARN_ON.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213203222.32564-2-matthew.auld@intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>