i915: Convert to use match_string() helper

The new helper returns index of the matching string in an array.
We are going to use it here.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503181706.22120-1-andriy.shevchenko@linux.intel.com