drm/xe: Convert memory device refcount to s32

The comparison with < 0 suggests that the memory device access
should be signed to handle underflow. This makes it work more reliably.

As a result, the max refcount is now S32_MAX instead.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>