Wait for POLLIN instead of POLLOUT

POLLIN waits for the buffer to be ready for reading, POLLOUT waits
for writing.

In gamescope we're reading client buffers, so we need to use POLLIN.

We were using POLLOUT to workaround an amdgpu bug, but that got
fixed in kernel 5.15, so shouldn't be necessary anymore.

src/steamcompmgr.cpp

@@ -511,7 +511,7 @@ retry:
 	assert( bFound == true );
 
 	gpuvis_trace_begin_ctx_printf( entry.commitID, "wait fence" );
-	struct pollfd fd = { entry.fence, POLLOUT, 0 };
+	struct pollfd fd = { entry.fence, POLLIN, 0 };
 	int ret = poll( &fd, 1, 100 );
 	if ( ret < 0 )
 	{