drm/msm: Convert to using __drm_atomic_helper_crtc_reset() for reset.

Convert msm to using __drm_atomic_helper_crtc_reset(), instead of
writing its own version. Instead of open coding
destroy_state(), call it directly for freeing the old state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
[mlankhorst: Remove double assignment of mdp5_cstate in reset (seanpaul)]
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-11-maarten.lankhorst@linux.intel.com