drm/i915: move watermark structs more towards usage

Shrink i915_drv.h a bit by moving watermark structs where they are
needed.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227170047.31089-3-jani.nikula@intel.com