drm/i915/region: convert object_create into object_init

Give more flexibility to the caller, if they already have an allocated
object, in case they wish to apply some transformation to the object
prior to handing it over to the region specific initialisation step,
like in gem_create_ext where we would like to first apply the extensions
to the object.

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/20210114182402.840247-3-matthew.auld@intel.com