drm/i915: Mark up inline getters as taking a const i915_request

Since these inline routines only return the desired pointer from the
i915_request(after checking the preconditions for acquiring said
pointer), they can be const.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616183139.4061-1-chris@chris-wilson.co.uk