dmi: Introduce the dmi_get_bios_year() helper function

dmi: Introduce the dmi_get_bios_year() helper function

The pattern to only extract the year portion of date is used in
several places and more users may come.

By using this helper they may create slightly cleaner code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[ Minor stylistic cleanup. ]
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jean Delvare <jdelvare@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-acpi@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Link: http://lkml.kernel.org/r/20180222125923.57385-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>