drm/rockchip: remove unused function

Fix the following clang warning:

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c:320:20: warning: unused
function 'dsi_set' [-Wunused-function].
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1618476421-114429-1-git-send-email-jiapeng.chong@linux.alibaba.com

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c

@@ -316,11 +316,6 @@ static inline u32 dsi_read(struct dw_mipi_dsi_rockchip *dsi, u32 reg)
 	return readl(dsi->base + reg);
 }
 
-static inline void dsi_set(struct dw_mipi_dsi_rockchip *dsi, u32 reg, u32 mask)
-{
-	dsi_write(dsi, reg, dsi_read(dsi, reg) | mask);
-}
-
 static inline void dsi_update_bits(struct dw_mipi_dsi_rockchip *dsi, u32 reg,
 				   u32 mask, u32 val)
 {