vc4: Warn instead of abort()ing on exec ioctl failures.

It's really harsh to abort() the X Server because of a momentary failure
(particularly -ENOMEM).  I don't see a way to pass an -ENOMEM up the stack
from here, but we can at least log to stderr before proceeding on.

Cc: "11.1" <mesa-stable@lists.freedesktop.org>

src/gallium/drivers/vc4/vc4_job.c

@@ -240,9 +240,11 @@ vc4_job_submit(struct vc4_context *vc4)
 #else
                 ret = vc4_simulator_flush(vc4, &submit);
 #endif
-                if (ret) {
-                        fprintf(stderr, "VC4 submit failed\n");
-                        abort();
+                static bool warned = false;
+                if (ret && !warned) {
+                        fprintf(stderr, "Draw call returned %s.  "
+                                        "Expect corruption.\n", strerror(errno));
+                        warned = true;
                 }
         }