p2sb: Factor out p2sb_read_from_cache()

To prepare for the following fix, factor out the code to read the P2SB
resource from the cache to the new function p2sb_read_from_cache().

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20241128002836.373745-2-shinichiro.kawasaki@wdc.com


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