avcodec: v4l2_m2m: context: fix raising warning on POLLERR

avcodec: v4l2_m2m: context: fix raising warning on POLLERR

During the initialization stage, the codec attempts to get free
buffers from the driver before any have been queued (this is to keep
the code simple and generic)

When the kernel driver detects this situation, it returns POLLERR in
revents and ffmpeg therefore raises a warning.

This commit disables the warning since no buffers were queued to the
driver yet.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>