This introduces a slight behavioral change to rmfb. Instead of
disabling a crtc when the primary plane is disabled, we try to
preserve it.

Apart from old versions of the vmwgfx xorg driver, there is
nothing depending on rmfb disabling a crtc.

Vmwgfx' and simple kms helper atomic implementation rejects CRTC
enabled without plane, so we can do this safely.

If the atomic commit is rejected by the driver then we will still
fall back to the old behavior and turn off the crtc.

Changes since v1:
- Restart completely when rmfb with crtc on fails (Sean Paul).
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@ubuntu.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101150433.10777-1-maarten.lankhorst@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>