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

drivers/gpu/drm/bridge/cdns-dsi.c: In function ‘cdns_dsi_mode2cfg’:
drivers/gpu/drm/bridge/cdns-dsi.c:515:11: warning: variable ‘nlanes’
set but not used [-Wunused-but-set-variable]

It is never used, and so can be removed.
Signed-off-by: yu kuai <yukuai3@huawei.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191226121415.39483-1-yukuai3@huawei.com