disp: msm: sde: poll for rsvp nxt after idle

During mode enable after idle, rsvp next would be set without
rsvp cur. Back to back test only should be serialized, but currently
this would be possible right after idle. This is established by
commit 11aab5f9 ("disp: msm: sde: Fix null dereference errors").
This change updates the condition to poll for rsvp next clear
during this usecase as well.

Change-Id: I300254967cbec69fff8b31545b8d659afc56f283
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>