drm/i915: Enforce that CS packets are qword aligned

We require the caller to ensure that the packets they wish to emit into
the CS ring are qword aligned (i.e. have an even number of dwords).
Double check this.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721161101.1618-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1712,6 +1712,9 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req,
 	unsigned int total_bytes;
 	u32 *cs;
 
+	/* Packets must be qword aligned. */
+	GEM_BUG_ON(num_dwords & 1);
+
 	total_bytes = bytes + req->reserved_space;
 	GEM_BUG_ON(total_bytes > ring->effective_size);