drm/i915: Use PIPE_CONF_CHECK_X() for sync_mode_slaves_mask

sync_mode_slaves_mask is a bitmask so use PIPE_CONF_CHECK_X() for it
so we get the mismatch printed in hex instead of decimal.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115190813.17971-4-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Tested-by: Manasi Navare <manasi.d.navare@intel.com>