drm/i915: extract fixed point math to i915_fixed.h

Reduce bloat in one of the bigger header files. Fix some indentation
while at it. No functional changes.

v2: Add include guards (Joonas)

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/20181116120729.7580-1-jani.nikula@intel.com