media: platform: mtk-mdp3: Use refcount_t for job_count

Use an API that resembles more the actual use of job_count.

Found by cocci:
drivers/media/platform/mediatek/mdp3/mtk-mdp3-cmdq.c:527:5-24: WARNING: atomic_dec_and_test variation before object free at line 541.
drivers/media/platform/mediatek/mdp3/mtk-mdp3-cmdq.c:578:6-25: WARNING: atomic_dec_and_test variation before object free at line 581.

Link: https://lore.kernel.org/linux-media/20240429-fix-cocci-v3-12-3c4865f5a4b0@chromium.org


Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>