drm/msm: Remove prototypes for non-existing functions

The msm_atomic_state_clear() and msm_atomic_state_free() functions are
declared but never defined. Remove their prototypes.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Patchwork: https://patchwork.freedesktop.org/patch/610618/


Signed-off-by: Rob Clark <robdclark@chromium.org>