[ALPS08745769] mt6989: disp: fix mode switch cmdq timeout

[Description]
When run below SW flow, it maybe occur cmdq timeout
step1: atomic_commit
step2: check trigger to set dirty
step3: mode switch: stop event loop --> set skip merge_num --> start event
loop, so we need to add stop trigger loop to avoid it

[Test]
MeStress

MTK-Commit-Id: 963b0be93c9f2079ae6abd8809bfd90547075d10

Signed-off-by: Chao Hao <chao.hao@mediatek.com>
CR-Id: ALPS08745769
Feature: display_mobile
Change-Id: I20dd8bdafec9d89a260f812f253ed44400b66303