drm/i915/guc: Dump the GuC stage descriptor pool in debugfs

We are missing pieces of information that could be useful for GuC
debugging.

v2: Reuse some code (Joonas)

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
[Joonas: Removed extra newline and s/uint32_t/u32/ for checkpatch.pl]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494428691-20672-1-git-send-email-oscar.mateo@intel.com