loader/dri3: Use strlen instead of sizeof for creating VRR property atom

sizeof counts the terminating null character as well, so that also
contributed to the ID computed for the X11 atom. But the convention is
for only the non-null characters to contribute to the atom ID.

Fixes: 2e12fe42 "loader/dri3: Enable adaptive_sync via
                     _VARIABLE_REFRESH property"
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>