i915: Move list_count() to list.h for broader use

i915: Move list_count() to list.h for broader use

Some of the existing users, and definitely will be new ones, want to
count existing nodes in the list. Provide a generic API for that by
moving code from i915 to list.h.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221123144901.40493-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>