mesa: align atomic buffer handling code with ubo/ssbo (v1.1)

mesa: align atomic buffer handling code with ubo/ssbo (v1.1)

this adds automatic size support to the atomic buffer code,
but also realigns the code to act like the ubo/ssbo code.

v1.1:
add missing blank lines.
reindent one block properly.
check for NullBufferObj.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>