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: 64761a841d "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>

src/gallium/state_trackers/va/picture.c

@@ -281,6 +281,7 @@ handleIQMatrixBuffer(vlVaContext *context, vlVaBuffer *buf)
 
    case PIPE_VIDEO_FORMAT_MPEG4:
       vlVaHandleIQMatrixBufferMPEG4(context, buf);
+      break;
 
    case PIPE_VIDEO_FORMAT_HEVC:
       assert(buf->size >= sizeof(VAIQMatrixBufferH264) && buf->num_elements == 1);