drm/i915: Don't restrict i915_wa_registers to BDW

We have CHV code that already makes the test obsolete. Besides, when
num_wa_regs is 0 (platforms not gathering that W/A data), we expose
something sensible already.

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>