msm: mdss: free smp reservations when rotator session is idle

Once rotation is done, the SMP allocations are no longer being used and
these blocks can be used by another client. This can also reduce the
memory being reserved if same pipe is used by multiple rotator sessions.

CRs-Fixed: 492113
Change-Id: I60e5c2111b2167395fc0d79ad40b2d0e7ec48086
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>