glx: lets compare drawing command sizes using MIN3

glx: lets compare drawing command sizes using MIN3

It has to fix coverity issue CID1470555:
```
481        if (bufSize > __GLX_RENDER_CMD_SIZE_LIMIT) {
482           bufSize = __GLX_RENDER_CMD_SIZE_LIMIT;
483        }
484        if (bufSize > __GLX_MAX_RENDER_CMD_SIZE) {
>>>     CID 1470555:  Control flow issues  (DEADCODE)
>>>     Execution cannot reach this statement: "bufSize = 64000;".
485           bufSize = __GLX_MAX_RENDER_CMD_SIZE;
486        }
```

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7965>