drm/i915/guc: Move enable/disable msg functions to GuC header

While today we are modifying GuC enabled msg mask only in GuC
log, this code should be defined as generic GuC to allow future
code reuse.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320181419.35576-3-michal.wajdeczko@intel.com