drm/vmwgfx: Use __drm_atomic_helper_crtc_reset

Now also comes with the added benefit of doing a drm_crtc_vblank_off(),
which means vblank state isn't ill-defined and fail-y at driver load
before the first modeset on each crtc.

v2: Compile fix. Oops.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612204940.2134653-1-daniel.vetter@ffwll.ch
Link: https://patchwork.freedesktop.org/patch/msgid/20200612160056.2082681-7-daniel.vetter@ffwll.ch