drm/i915: add register macro definition style guide

This is not to try to force a new style; this is my interpretation of
what the most common existing style is.

With hopes I don't need to answer so many questions about style going
forward.

Start a new style section in the i915 document to bolt the register
style guide into.

v2: vertical alignment, incorporate to kernel-doc, and more

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9de4a5b1bea4e76461c70a1dd66751581de0124f.1502368010.git.jani.nikula@intel.com