drm/i915/gem: Refactor placement setup for i915_gem_object_create* (v2)

Since we don't allow changing the set of regions after creation, we can
make ext_set_placements() build up the region set directly in the
create_ext and assign it to the object later.  This is similar to what
we did for contexts with the proto-context only simpler because there's
no funny object shuffling.  This will be used in the next patch to allow
us to de-duplicate a bunch of code.  Also, since we know the maximum
number of regions up-front, we can use a fixed-size temporary array for
the regions.  This simplifies memory management a bit for this new
delayed approach.

v2 (Matthew Auld):
 - Get rid of MAX_N_PLACEMENTS
 - Drop kfree(placements) from set_placements()
v3 (Matthew Auld):
 - Properly set ext_data->n_placements

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723172142.3273510-3-jason@jlekstrand.net