intel: Fix Haswell GT3 names.

When publishing first HSW ids we weren't allowed to use "GT3" codname.
But this is the correct codname and Mesa is using it already.
So to avoid people getting confused why in Mesa it is called GT3 and here
it is called GT2_PLUS let's fix this name in a standard and correct way.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>