drm/rockchip: dsi: Remove the unused function dsi_read()

The function dsi_read is defined in the dw-mipi-dsi-rockchip.c file, but
not called elsewhere, so remove this unused function.

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c:362:19: warning: unused function 'dsi_read'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3984


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/20230208023429.73696-1-jiapeng.chong@linux.alibaba.com