radv: Align large buffers to the fragment size.

Improves performance in Talos by about 15% (and significant improvements
in RotR and possibly other but did not bench with final patch) on
kernel 4.19 and earlier.

On 4.20+ a similar effect comes from

433ca054949a "drm/amdgpu: try allocating VRAM as power of two"

v2: Do not impact the alignment of the physical memory.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 6569644b)