drm/i915/fixed: prefer kernel types over stdint types

While at it, conform to kernel spacing (i.e. no space) after cast. 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/20181116120729.7580-2-jani.nikula@intel.com