drm/i915/region: don't leak the object on error

Sanity check the object size before allocating a new gem object.

Fixes: 97d55396 ("drm/i915/region: convert object_create into object_init")
Testcase: igt/gem_create/create-massive
Signed-off-by: Matthew Auld <matthew.auld@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/20210120104714.112812-1-matthew.auld@intel.com