drm/i915: abstract and document register picking macros

Try to describe what the pick variants do, and which to prefer. No
functional changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180629102039.2435-1-jani.nikula@intel.com