drm/i915: rename INSTDONE1 to GEN4_INSTDONE1

This register was added on GEN4, by the name INSTDONE_1 whereas the GEN6
specification calls it INSTDONE_2. Keep the original name with a
platform prefix to make it clearer which INSTDONE register instance this
is. Also add a comment about the SNB alternative name.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>