drm/amd/display: remove unused function setFieldWithMask

After commit a9f54ce3

 ("drm/amd/display: Refactoring VTEM"),
there is no caller in tree.

Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>