x86, perf: Fix few cosmetic dabs for P4 pmu (comments and constantify)

- A few ESCR have escaped fixing at previous attempt.
- p4_escr_map is read only, make it const.

Nothing serious.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Lin Ming <ming.m.lin@intel.com>
LKML-Reference: <20100318211256.GH5062@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>