video: fbdev: omapfb: Fix set but not used warnings in dsi

Fix several W=1 warnings.
This removes unused code and avoids an assignment by moving
the use inside the conditional block.

The register read FLD_GET(r, 15, 8) could be dropped as it was done a
few lines before too.

v2:
  - Updated subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Aditya Pakki <pakki001@umn.edu>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-23-sam@ravnborg.org