freedreno: Fix data races with allocating/freeing struct ir3.

There is a single ir3_compiler in the screen, and each context may be
compiling ir3 shaders, which call ir3_create.  ralloc doesn't do any
locking on its own, so eventually you can end up racing to break
ralloc's linked lists.

We really don't want struct ir3 to live as long as the compiler (maybe
struct ir3_shader's lifetime, if anything), so you'd better be freeing
it anyway.

Fixes: 8fe20762 ("freedreno/ir3: convert over to ralloc")
Reviewed-by: Rob Clark <robdclark@gmail.com>
(cherry picked from commit 6e3b220a)