[ALPS08758947] mt6878: disp: skip clear_layer when wcg force gpu

[Description]
Skip clear_layer flow if layer is labeled as wcg_force_gpu,
to avoid this layer be process by dev again.

[Test]
MEStress, CTS test

MTK-Commit-Id: e63268e6744d999e2e66493fac290fff95dfae31

Signed-off-by: kai.shi <kai.shi@mediatek.com>
CR-Id: ALPS08758947
Feature: display_mobile
Change-Id: I58d810c558fb9031a132b136d21e6cc76d176b66