drm/i915/uc: Introduce intel_uc_suspend|resume

We want to use higher level 'uc' functions as the main entry points to
the GuC/HuC code to hide some details and keep code layered.

While here, move call to disable_guc_interrupts after sending suspend
action to the GuC to allow it work also with CTB as comm mechanism.

v2: update commit msg (Sagar)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302111550.21328-1-michal.wajdeczko@intel.com