drm/ci: reduce sm8350-hdk parallel jobs from 4 to 2

The sm8350-hdk jobs are short and each test takes around 2–3 minutes and
the full job completes in about 10 minutes. Running 4 parallel jobs uses
4 devices at once, which is not needed. Set parallel to 2 to reduce
device usage.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>