drm/i915: Per-engine scratch VMA is mandatory

We fail engine initialization if the scratch VMA cannot be created so
there is no point in error handle it later. If the initialization ordering
gets messed up, we can explode during development just as well.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180119100005.9072-2-tvrtko.ursulin@linux.intel.com