platform/x86/amd/pmf: Initialize and clean up `cb_mutex`

`cb_mutex` was introduced in commit 9e0894d0 ("platform/x86/amd/pmf:
Enable Custom BIOS Inputs for PMF-TA") to prevent conccurrent access for
BIOS inputs. It however isn't initialized and so on use it may lead to
a NULL pointer dereference.

Add code to initialize on probe and clean up on destroy.

Reported-by: Yijun Shen <Yijun.Shen@dell.com>
Cc: Richard Gong <richgong@amd.com>
Fixes: 9e0894d0 ("platform/x86/amd/pmf: Enable Custom BIOS Inputs for PMF-TA")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-By: Yijun Shen <Yijun_Shen@Dell.com>
Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20250227170308.435862-1-superm1@kernel.org


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>