drm/i915/guc: Refactoring preparation of the GUC_CTL_CTXINFO parameter

At the moment, the preparation of GUC_CTL_CTXINFO is disordered.
Lets move all  GUC_CTL_CTXINFO related operations to one place.

v2:
- move 'ctxnum' and 'base' declarations to USES_GUC_SUBMISSION case
(Michał Wajdeczko)

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180604141947.8299-5-piotr.piorkowski@intel.com