drm/i915: rename INSTDONE to GEN2_INSTDONE

We have a bunch of INSTDONE registers for different platforms and
purposes and it's not immediately clear which instance they are just by
looking at the register name. This one was added on GEN2, where it was
the only INSTDONE register, so mark it as such.

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>