drm: prevent a harmless integer overflow in drm_legacy_sg_alloc()

There is an integer overflow when we round up to PAGE_SIZE, but it's
harmless because we never re-use "request->size" for anything meaningful.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228092321.axulddmkxrujkmas@kili.mountain