disp: msm: sde: avoid multiple frame-done encoder events

Currently there is a race condition in checking the
pending_kickoff_cnt in wr_ptr_irq wait from display-thread
and pp_done_irq from interrupt context. In both places,
pending_kickoff_cnt is read first and modified later. In
partial update cases where the frame-transfer is short,
such a race condition might happen and would lead to both
triggering the frame-done/release fence for the same frame.
Fix it by combining read/modify to one statement in both places.

Change-Id: I9162e7dc3f12af3590514f1ebfd68023aa920181
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>