drm/msm/mdp5: drop single flush support

drm/msm/mdp5: drop single flush support

Support for using a single CTL for flushing both interfaces was not in
use since the MDP5 driver dropped support for dual DSI configurations in
the commit df3c7899946c ("drm/msm/mdp5: drop split display support").
Having the MDP 3.x support migrated to the DPU driver the single CTL
flush is applicable to the platforms suspproted by the MDP5 driver. Drop
it alltogether.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/713916/
Link: https://lore.kernel.org/r/20260325-mdp5-drop-single-flush-v1-1-862a38b4d2ec@oss.qualcomm.com