freedreno: Fix assertion failures in context setup in shader-db mode.

Cherry-picks a0d4d7fe upstream

The TTN path needs access to the screen to make the right decisions about
lowering, but we didn't have pctx->screen set up at fdN_prog_init time.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Eduardo Lima Mitev <elima@igalia.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>