st/mesa: Fix signed integer overflow when using util_throttle_memory_usage

st/mesa: Fix signed integer overflow when using util_throttle_memory_usage

../src/mesa/state_tracker/st_cb_texture.c:1719:57: runtime error: signed integer overflow: 203489280 * 16 cannot be represented in type 'int'

Fixes: 21ca322e637291b89a445159fc45b8dbf638e6c9
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4185>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4185>