fix: 'm_state' was not declared in this scope

Error occurred when building with DEBUG_PLAYBIN defined.
It should be m_currentState from context.

Change-Id: Iad59966ca19c9d1b589fc8641599494436a0a5f3
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Jing Bai <doris.bai@gmail.com>