No need to list all the platforms explicitly.

The prefix is a bit inconsistent since we usually pick gen8_ for GT
related functions. But this anti-pattern is already established with snb,
so material for a different patch.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>