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

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 <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Chris Wilson <[email protected]>
Reviewed-by: Michal Wajdeczko <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]