drm/i915/guc: Move check for fast memcpy_wc to relay creation

We only need those fast memcpy_wc when we're using relay to read
continuous GuC log. Let's prevent the user from creating a relay if we
know we won't be able to keep up with GuC.

v2: Adjust the return value (Michał)

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
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/20180319095348.9716-6-michal.winiarski@intel.com