drm/i915: rename IOSF sideband opcodes according to the spec

These opcodes are not specific for an endpoint, but are the same for all
endpoints. So rename them accordingly, using the name the VLV2 sideband
HAS uses. Also move the macros to the .c file, since they aren't used
anywhere else.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>