mdss: rotator: validate rotator work item against session

The rationale for open session is to accurately vote for bw and
clk rates based on configuration provided. Prior to this patch it
was possible to call open session with one configuration, but
provided an entirely different rotation configuration as work to
be completed.

Change-Id: I82c95c290cffa75ea7b09ea4a021e9e436782c06
Signed-off-by: Terence Hampson <thampson@codeaurora.org>