drm/msm/mdp5: Avoid flushing registers when CRTC is disabled

When a CRTC is disabled, no CTL is allocated to it (CRTC->ctl == NULL);
in that case we should not try to FLUSH registers and do nothing instead.

This can happen when we try to move a cursor but the CRTC's CTL
(CONTROL) has not been allocated yet (inactive CRTC).
It can also happens when we .atomic_check()/.atomic_flush() on a
disabled CRTC.

A CTL needs to be kept as long as the CRTC is alive. Releasing it
after the last VBlank is safer than in .atomic_flush().

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>