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

drivers/gpu/drm/omapdrm/dss/dsi.c

@@ -3548,7 +3548,7 @@ static int dsi_proto_config(struct dsi_data *dsi)
 
 static void dsi_proto_timings(struct dsi_data *dsi)
 {
-	unsigned int tlpx, tclk_zero, tclk_prepare, tclk_trail;
+	unsigned int tlpx, tclk_zero, tclk_prepare;
 	unsigned int tclk_pre, tclk_post;
 	unsigned int ths_prepare, ths_prepare_ths_zero, ths_zero;
 	unsigned int ths_trail, ths_exit;
@@ -3567,7 +3567,6 @@ static void dsi_proto_timings(struct dsi_data *dsi)
 
 	r = dsi_read_reg(dsi, DSI_DSIPHY_CFG1);
 	tlpx = FLD_GET(r, 20, 16) * 2;
-	tclk_trail = FLD_GET(r, 15, 8);
 	tclk_zero = FLD_GET(r, 7, 0);
 
 	r = dsi_read_reg(dsi, DSI_DSIPHY_CFG2);