xvmc: fix string comparison

Fixes: c7b65dca "xvmc: Define some Xv attribs to allow users
                             to specify color standard and procamp"
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 110a6e18)