drivers/mipi: sta_himax83102: Completely pull GPW to VGL before TP term

The sta_himax83102 panel sometimes shows abnormally flickering
horizontal lines. The front gate output will precharge the X point of
the next pole circuit before TP term starts, and wait until the end of
the TP term to resume the CLK. For this reason, the X point must be
maintained during the TP term. In abnormal case, we measured a slight
leakage at point X. This is because during the TP term, the GPW does not
fully pull the VGL low, causing the TFT to not be closed tightly.

To fix this, we completely pull GPW to VGL before entering the TP term.
This will ensure that the TFT is closed tightly and prevent the abnormal
display.

BUG=b:299249186
BRANCH=corsola
TEST=FW Screen display normally

Signed-off-by: Ruihai Zhou <zhouruihai@huaqin.corp-partner.google.com>
Change-Id: I5dddaaa38917a65990c1474b657db5eb551940b1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77692
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Yidi Lin <yidilin@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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