msm: sde: avoid redundant signal/release on finished request

Finished request is already signaled and released in done handler.
Undispatched request is signaled and released during session
close by cancel request. However, cancel request does not
check if a request is finished or not, and will trigger
signal and release even if the request is already handled
by done handler. This generates misleading information on
session close.

CRs-Fixed: 2009714
Change-Id: I81c376396741d978f95c7161183dfb7092075833
Signed-off-by: Alan Kwong <akwong@codeaurora.org>