drm/i915: remove duplicate names for the render ring INSTDONE register

We use 3 different names to refer to the same render ring INSTDONE
register. This can be confusing when comparing two parts of the code
accessing the register via different names. Although the GEN4 version's
layout is different, we treat it the same way as the GEN7+ version, in
that we simply read it out during error capture. So remove the
duplicates and leave a comment about the GEN4 difference.

Note that there is also a GEN2 version of this register, but that's on a
different address so not handled in this patch.

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>