msm: mdss: Return ESHUTDOWN error when driver is shutting down

When MDP driver is being shutdown, driver clients can make ioctl calls.
Driver was returning a generic error which results in clients
re-issuing the ioctl. Driver should return ESHUTDOWN error to notify
the clients that it is shutting down and cannot accept any ioctls.
Change returns the shutdown error for ioctls issued during shutdown
sequence.

Change-Id: Ie5a8e557565feb50edd858f9a2ee98637efb55d0
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>