drm/i915/guc: Init send_mutex in intel_uc_init_early()

send_mutex is used to serialise communication with GuC via
intel_guc_send().

Since functions that utilize it are no longer limited to submission,
initialization should be handled as a part of general setup.

v2: move initialization to *_early()

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1480096777-12573-5-git-send-email-arkadiusz.hiler@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>