disp: msm: sde: update rsvp_cur when poll is enabled for rsvp_nxt

This change updates rsvp_cur pointer to latest to avoid use
after free issues. rsvp_cur pointer can be freed in few cases due
to the unlock, wait and lock of rm_lock present in
_sde_rm_poll_get_rsvp_nxt_locked.

Change-Id: I389048188e8a615edc3e75dd1102d4ca8c74af65
Signed-off-by: Jayaprakash Madisetty <jmadiset@codeaurora.org>