disp: msm: sde: fix tracking of pending rsvps

Previous logic expected rsvp_nxt to always be the 2nd rsvp for
a given encoder. This does not work for releasing back-to-back
test-only reservations (with no current rsvp). If new
resources become available between the 2 test-only commits we
may end up reserving more resources than necessary. Change the
logic by adding a pending flag to the rsvp to simplify lookup
of both current and pending/next resource reservations.

Change-Id: Iefa8085f1b4c0cf49bb744dcc2deb7fb92608454
Signed-off-by: Steve Cohen <cohens@codeaurora.org>