drm: fix uninitialized acquire_ctx fields (v2)

The acquire ctx will typically be declared on the stack, which means we
could have garbage values for any uninitialized field.  In this case, it
was triggering WARN_ON()s because 'contended' had garbage value.

Go ahead and use memset() to be more future-proof.

v2: now with extra brown paper bag
Reported-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NRob Clark <robdclark@gmail.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_modeset_lock.c

@@ -64,6 +64,7 @@
 void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
 		uint32_t flags)
 {
+	memset(ctx, 0, sizeof(*ctx));
 	ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
 	INIT_LIST_HEAD(&ctx->locked);
 }