Extract the part setting up from xe->info from xe_pci_probe() into its
own function. This pairs nicely with the display counterpart, avoids
info initialization to be placed elsewhere and helps future
improvements to build fake devices for tests.

While at it, normalize the names a little bit: the _get() suffix may be
mistaken by lock-related operation, so rename the function to
"find_subplatform()". Also rename the variable to subplatform_desc to
make it easier to understand, even if longer.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230401085151.1786204-2-lucas.demarchi@intel.com