oprofile/x86: reserve counter msrs pairwise

For AMD's and Intel's P6 generic performance counters have pairwise
counter and control msrs. This patch changes the counter reservation
in a way that both msrs must be registered. It joins some counter
loops and also removes the unnecessary NUM_CONTROLS macro in the AMD
implementation.

Signed-off-by: Robert Richter <robert.richter@amd.com>