perf/x86/amd/brs: Move feature-specific functions

Move some of the Branch Sampling (BRS) specific functions out of the Core
events sources and into the BRS sources in preparation for adding other
mechanisms to record branches.

Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/b60283b57179475d18ee242d117c335c16733693.1660211399.git.sandipan.das@amd.com