drm/i915: Don't overrun the intel_wa_regs array

When entering intel_ring_emit_wa() with num_wa_regs equal to
I915_MAX_WA_REGS, we end up indexing the intel_wa_regs array beyond its
allocation.

Fix the check then.

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>