st/va: add missing break statement

st/va: add missing break statement

Earlier commit factored out the mpeg4 IQ matrix handling into separate
function, although it forgot to add a break in its case statement.
Thus the data ended up partially overwritten as the mpeg4 and h265
structs are members of the desc union.

Spotted by Coverity (CID 1341052)

Fixes: 64761a841db "st/va: move MPEG4 functions into separate file"
Cc: Julien Isorce <j.isorce@samsung.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>