display/blizzard: use extract16() for fix clang analyzer warning in blizzard_draw_line16_32()

Clang static code analyzer show warning:
  hw/display/blizzard.c:940:9: warning: Value stored to 'data' is never read
        data >>= 5;
        ^        ~
Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200325025919.21316-3-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>