drm/i915/guc: Extract clients allocation to submission_init

drm/i915/guc: Extract clients allocation to submission_init

We can now move the clients allocation to submission_init path, rather
than keeping the condition inside submission_enable called on every
reset.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213221352.7173-6-michal.winiarski@intel.com