drm/vmwgfx/stdu: Use drm_mode_config_reset

When converting to atomic the state reset was done by directly calling
the functions, and before the modeset object was fully initialized.
This means the various ->dev pointers weren't set up.

After

commit 51f644b4
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jun 12 18:00:49 2020 +0200

    drm/atomic-helper: reset vblank on crtc reset

this started to oops because now we're trying to derefence
drm_crtc->dev. Fix this up by entirely switching over to
drm_mode_config_reset, called once everything is set up.

Fixes: 51f644b4 ("drm/atomic-helper: reset vblank on crtc reset")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Tested-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>