drm/i915: Rename intel_wa_registers with a i915_ prefix

Those debugfs files are prefixed by i915, the name of the kernel module,
presumably to make the difference with files exposed by core DRM.

Also, add a ',' at the end of the last entry. This is to ease the
conflict resolution when rebasing internal patches that add a member at
the end of the array. Without it, wiggle can't do its job as we need to
modify an existing line (appending the ',').

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>