drm/msm/dpu: Always flush the slave INTF on the CTL

[ Upstream commit 2b938c3a ]

As we can clearly see in a downstream kernel [1], flushing the slave INTF
is skipped /only if/ the PPSPLIT topology is active.

However, when DPU was originally submitted to mainline PPSPLIT was no
longer part of it (seems to have been ripped out before submission), but
this clause was incorrectly ported from the original SDE driver.  Given
that there is no support for PPSPLIT (currently), flushing the slave
INTF should /never/ be skipped (as the `if (ppsplit && !master) goto
skip;` clause downstream never becomes true).

[1]: https://git.codelinaro.org/clo/la/platform/vendor/opensource/display-drivers/-/blob/display-kernel.lnx.5.4.r1-rel/msm/sde/sde_encoder_phys_cmd.c?ref_type=heads#L1131-1139

Fixes: 25fdd593

 ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/589901/
Link: https://lore.kernel.org/r/20240417-drm-msm-initial-dualpipe-dsc-fixes-v1-3-78ae3ee9a697@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>