drm/msm/dpu: don't set crtc_state->mode_changed from atomic_check()

drm/msm/dpu: don't set crtc_state->mode_changed from atomic_check()

The MSM driver uses drm_atomic_helper_check() which mandates that none
of the atomic_check() callbacks toggles crtc_state->mode_changed.
Perform corresponding check before calling the drm_atomic_helper_check()
function.

Fixes: 8b45a26f2ba9 ("drm/msm/dpu: reserve cdm blocks for writeback in case of YUV output")
Reported-by: Simona Vetter <simona.vetter@ffwll.ch>
Closes: https://lore.kernel.org/dri-devel/ZtW_S0j5AEr4g0QW@phenom.ffwll.local/
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
[DB: dropped the WARN_ON]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/633400/
Link: https://lore.kernel.org/r/20250123-drm-dirty-modeset-v2-4-bbfd3a6cd1a4@linaro.org