drm/i915/gt: Export the pinned context constructor and destructor

Allow internal clients to create and destroy a pinned context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617063018.92802-7-thomas.hellstrom@linux.intel.com