disp: msm: sde: clear partial update mask during last close

Clear partial update mask during last close to avoid
rounded corner to be programmed without valid mask during
shell stop/start operations.

Change-Id: I378adc62b5bc5913abfc32fa3528c8e28ea273d4
Signed-off-by: Anjaneya Prasad Musunuri <aprasad@codeaurora.org>