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

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

Convert vkms 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: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-17-maarten.lankhorst@linux.intel.com
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>