drm/i915/ttm: fix large buffer population trucation

ttm->num_pages is uint32_t which was causing very large buffers to
only populate a truncated size.

This fixes gem_create@create-clear igt test on large memory systems.

Fixes: 7ae03459 ("drm/i915/ttm: add tt shmem backend")
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211210195005.2582884-1-bob.beckett@collabora.com