powercap: intel_rapl: Fix a NULL pointer dereference

A NULL pointer dereference is triggered when probing the MMIO RAPL
driver on platforms with CPU ID not listed in intel_rapl_common CPU
model list.

This is because the intel_rapl_common module still probes on such
platforms even if 'defaults_msr' is not set after commit 1488ac99
("powercap: intel_rapl: Allow probing without CPUID match"). Thus the
MMIO RAPL rp->priv->defaults is NULL when registering to RAPL framework.

Fix the problem by adding sanity check to ensure rp->priv->rapl_defaults
is always valid.

Fixes: 1488ac99 ("powercap: intel_rapl: Allow probing without CPUID match")
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Cc: 6.5+ <stable@vger.kernel.org> # 6.5+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>