drm/msm/dsi: Remove set but not used variable 'lp'

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

drivers/gpu/drm/msm/dsi/dsi_host.c: In function dsi_cmd_dma_rx:
drivers/gpu/drm/msm/dsi/dsi_host.c:1302:7: warning: variable lp set but not used [-Wunused-but-set-variable]

It is not used since commit a689554b ("drm/msm:
Initial add DSI connector support")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1570690506-83287-5-git-send-email-zhengbin13@huawei.com