video: fbdev: omapfb: Fix set but not used warnings in hdmi*_core

Fix a few W=1 warnings about unused assignments.
Drop the unused error code.

v2:
  - Subject updated (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Qilong Zhang <zhangqilong3@huawei.com>
Cc: "Alexander A. Klimov" <grandmaster@al2klimov.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-4-sam@ravnborg.org