freedreno: allow ctx->batch to be NULL

freedreno: allow ctx->batch to be NULL

This was mostly true already, now that we use `fd_context_batch()` for
first access to batch in draw/clear/grid paths.  So we can drop the old
code in `batch_flush()` that tried to prevent `ctx->batch` from being
NULL.

Fixes a crash with a large number of tabs in chromium.

Cc: "20.0" mesa-stable@lists.freedesktop.org
Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3700>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3700>