drm/i915/fixed: simplify FP_16_16_MAX definition

No need to use a compound statement enclosed in parenthesis where a C99
compound literal will do. No functional changes.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181116120729.7580-3-jani.nikula@intel.com