drm/i915: remove unused/stale macros and comments from intel_drv.h

Reduce clutter from intel_drv.h.

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/963ba7fa0111135c3e796bfc9f86d6e33724758e.1556809195.git.jani.nikula@intel.com