lavf/qsv_vpp: check the return value of ff_formats_ref()

Fixes the build warning of "ignoring return value of ‘ff_formats_ref’,
declared with attribute warn_unused_result"

Signed-off-by: Zhong Li <zhong.li@intel.com>
Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>
