Only constbufs must be aligned to 0x100, but since all buffers can be
rebinded as constant buffers they must be also aligned.

This patch prevents this behaviour by aligning everything to 256-byte
increments at buffer creation.

This fixes dmesg fails for the following piglit test:
  ext_transform_feedback-immediate-reuse-uniform-buffer -auto -fbo
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>