drm/i915: Use correct reST syntax for WOPCM and GuC kernel-doc diagrams

GuC Address Space and WOPCM Layout diagrams won't be generated correctly by
sphinx build if not using proper reST syntax.

This patch uses reST literal blocks to make sure GuC Address Space and
WOPCM Layout diagrams to be generated correctly, and it also corrects some
errors in the diagram description.

v2:
 - Fixed errors in diagram description

v3:
 - Updated GuC Address Space kernel-doc based on Michal's suggestion

v4:
 - Added WOPCM layout and GuC address space docs into i915.rst (Joonas)

Signed-off-by: Jackie Li <yaodong.li@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1521763162-11424-1-git-send-email-yaodong.li@intel.com