drm/i915: Drop /** */ comments from i915_reg.h

The comments in i915_reg.h aren't proper kernel-doc comments, so replace
the magic /** with just /*

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>