[PATCH v3] drm/mediatek: Fix unused-but-set variable warning

[miles.chen at mediatek.com]

> I'm still not happy with the commit subject, I think it is misleading. Clang 
> only helped to find the bug, but the we are fixing something else, that's not 
> just a clang warning. But I don't want to nit-pick too much so:
> 
> Reviewed-by: Matthias Brugger <matthias.bgg at gmail.com>

thanks. I think you are right.
I will change the subject to "drm/mediatek: Fix mtk_cec_mask()", remove the 
clang part and submit patch v4.

e.g,
"""
drm/mediatek: Fix mtk_cec_mask()

In current implementation, mtk_cec_mask() writes val into target register
and ignores the mask. After talking to our hdmi experts, mtk_cec_mask()
should read a register, clean only mask bits, and update (val | mask) bits
to the register.

Fixes: 8f83f26891e1 ("drm/mediatek: Add HDMI support")
"""