radv: Prevent out of bound shift on 32-bit builds.

uintptr_t is 32-bits then and shifting it by 32 bits results in undefined
behavior IIRC.

Fixes: b3c8de1c "radv: save all descriptor pointers into the trace BO"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 39c71e00)