drm/omap: Remove set but not used variable 'plane'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/omapdrm/omap_fb.c: In function omap_framebuffer_update_scanout:
drivers/gpu/drm/omapdrm/omap_fb.c:130:16: warning: variable plane set but not used [-Wunused-but-set-variable]

It is not used since commit 2ecceeb5

 ("drm/omap:
Move buffer pitch/offset to drm_framebuffer")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1570518949-47574-2-git-send-email-zhengbin13@huawei.com