drm/msm/dpu: fix encoder irq wait skip

drm/msm/dpu: fix encoder irq wait skip

The irq_idx is unsigned so it cannot be lower than zero, better
to change the condition to check if it is equal with zero.
It could not cause any issue because a valid irq index starts from one.

Fixes: 5a9d50150c2c ("drm/msm/dpu: shift IRQ indices by 1")
Signed-off-by: Barnabás Czémán <trabarni@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/596853/
Link: https://lore.kernel.org/r/20240509-irq_wait-v2-1-b8b687b22cc4@gmail.com
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>