avcodec/hevcdec: hevc_await_progress: declare |y| only if used.

hevc_await_progress() uses the variable |y| only inside the "if" block.
So |y| only needs to be declared and initialized in that block.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
