perfmon: refactor perf metric support

perfmon: refactor perf metric support

Refactoring perf metric support to remove branching on bundle type in
the dispatch wrapper. This change includes caching the rdpmc index at
perfmon_start(), so that the mmap_page.index doesn't need to be looked
up each time. It also exclude the effects of mmap_page.index.

This patch prepares the path for bundles that support general, fixed and
metrics counters simulataneously.

Type: refactor

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>
Change-Id: I9c5b4917bd02fea960e546e8558452c4362eabc4