drm/vtables: Fix typo

The callback is named .atomic_check, not .atomc_check.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901144042.6023-1-thierry.reding@gmail.com