drm/i915: add HAS_ASYNC_FLIPS feature macro

This will be needed in multiple places soon.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/08bf0f72435a4f9acb0ef31b82ca312b048c6bf6.1629721467.git.jani.nikula@intel.com