ci/freedreno: Stop specifying the number of deqp threads

For "0" (its default) deqp-runner picks a number of jobs corresponding to
the CPU count, so set our hardware runners to use that (note that mesa's
deqp-runner.sh will pick a default of 4 if we don't specify a
DEQP_PARALLEL).

This means we'll allocate threads for the slow cores on a630 now, reducing
gles3 runtime from 6.5 minutes to around 5.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8132>

.gitlab-ci.yml

@@ -1329,8 +1329,8 @@ arm64_a306_gles2:
     DEQP_EXPECTED_FAILS: deqp-freedreno-a307-fails.txt
     DEQP_SKIPS: deqp-default-skips.txt
     DEQP_FLAKES: deqp-freedreno-a307-flakes.txt
+    DEQP_PARALLEL: 0 # Default to number of CPUs
     DEQP_VER: gles2
-    DEQP_PARALLEL: 4
     DEQP_EXPECTED_RENDERER: FD307
   script:
     - ./install/bare-metal/fastboot.sh