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: 8b45a26f ("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>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>