msm: mdss: Introduce error checks for rotator sessions

When there are more virtual rotator sessions allocated than
DMA pipes available, rotator pipe allocations might fail and
corresponding error messages are printed which can lead to a
watchdog bark. This change silences those error messages and
also implements an extra check to avoid null pointer
dereference when a session is dequeued without a pipe.

CRs-fixed: 523701
Change-Id: I128ab85aac9e091b13a8772a43342d655c6f736f
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>