drm/i915: fix wrong error number report

During driver load it's considered that the i915_driver_create()
function fails only in case of insufficient memory. Indeed, in
case of failure of i915_driver_create(), the load function
returns indiscriminately -ENOMEM ignoring the real cause of
failure.

In i915_driver_create() get the consistent error value from
drm_dev_init() and embed it in the pointer return value.

Signed-off-by: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002092047.14705-1-andi.shyti@intel.com