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>