drm/i915/overlay: Stash the kernel context on initialisation

Simplify runtime request creation by storing the context we need to use
during initialisation. This allows us to remove one more hardcoded
engine lookup.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704200455.14870-1-chris@chris-wilson.co.uk