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

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

drivers/gpu/drm/omapdrm/dss/dsi.c: In function dsi_proto_timings:
drivers/gpu/drm/omapdrm/dss/dsi.c:3562:46: warning: variable tclk_trail set but not used [-Wunused-but-set-variable]

It is not used since commit 9960aa7c

 ("drm/omap:
move omapdss & displays under omapdrm")

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/1570518949-47574-3-git-send-email-zhengbin13@huawei.com