drm/i915/stolen: shuffle around init_memory_region

We generally want to first call i915_gem_object_init_memory_region()
before calling into get_pages(), since this sets up various bits of
state which might be needed there. Currently for stolen this doesn't
matter much, but it might in the future, and at the very least this
makes things consistent with the other backends.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507095948.384230-1-matthew.auld@intel.com