Fixed decoder shutdown freeze with MPEG TS h.264 content and qtquick2videosink

When setting pipeline to NULL it gets stuck in the imxvpudec. Hooking
into the state change on PAUSED to READY by calling gst_imx_vpu_dec_flush()
solves the issue.
Modified decoder shutdown freeze fix

Using gst_imx_vpu_framebuffers_set_flushing() instead of
gst_imx_vpu_dec_flush(), since the former keeps the condition variable
deactivated, while the latter does so only temporarily

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>