drm/msm: Remove unused pm_state

This was added in commit ec446d09 ("drm/msm: call
drm_atomic_helper_suspend() and drm_atomic_helper_resume()"), but unused
since commit ca8199f1 ("drm/msm/dpu: ensure device suspend happens
during PM sleep") which switched to drm_mode_config_helper_suspend()/
drm_mode_config_helper_resume()..

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/607746/