perf/x86/rapl: Make rapl_model struct global

Prepare for the addition of RAPL core energy counter support.

As there will always be just one rapl_model variable on a system, make it
global, to make it easier to access it from any function.

No functional change.

Signed-off-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: "Gautham R. Shenoy" <gautham.shenoy@amd.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20241115060805.447565-6-Dhananjay.Ugwekar@amd.com