video: fbdev: nvidia: Fix set but not used warnings

Fix warnings by deleting unused code. The register reads are
kept as it is unknown if there are any hidden side-effects.

v2:
  - Update subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Antonino Daplas <adaplas@gmail.com>
Cc: linux-fbdev@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-21-sam@ravnborg.org