msm: sde: Avoid NULL pointer dereference in cancel request

There is a race condition possible when two threads are calling
the rotator cancel request. This might result in accessing a pointer
which was already assigned NULL. Fixing this by adding an extra check.

Change-Id: I9ce321a5f033d1fdc9d8b70a04098bfba3d7baaa
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>