drm/nouveau: Fix fallout from reservation object rework

Commit 019cbd4a ("drm/nouveau: Initialize GEM object before TTM
object") introduced a subtle change in how the buffer allocation size is
handled. Prior to that change, the size would get aligned to at least a
page, whereas after that change a non-page-aligned size would get passed
through unmodified. This ultimately causes a BUG_ON() to trigger in
drm_gem_private_object_init() and crashes the system.

Fix this by restoring the code that align the allocation size.

Fixes: 019cbd4a ("drm/nouveau: Initialize GEM object before TTM object")
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>