MLK-22304-1 drm/imx: lcdif-dsim: fix build warnings if CONFIG_PM_SLEEP off

MLK-22304-1 drm/imx: lcdif-dsim: fix build warnings if CONFIG_PM_SLEEP off

When CONFIG_PM_SLEEP is disabled, the suspend and resume hooks
are implemented as dummy functions, but GCC will report below
build warnings:

drivers/gpu/imx/lcdif/lcdif-common.c:731:12: warning: ‘imx_lcdif_suspend’ defined but not used [-Wunused-function]
 static int imx_lcdif_suspend(struct device *dev)
            ^
drivers/gpu/imx/lcdif/lcdif-common.c:735:12: warning: ‘imx_lcdif_resume’ defined but not used [-Wunused-function]
 static int imx_lcdif_resume(struct device *dev)
            ^
drivers/gpu/drm/imx/sec_mipi_dsim-imx.c:324:12: warning: ‘imx_sec_dsim_suspend’ defined but not used [-Wunused-function]
 static int imx_sec_dsim_suspend(struct device *dev)
            ^
drivers/gpu/drm/imx/sec_mipi_dsim-imx.c:329:12: warning: ‘imx_sec_dsim_resume’ defined but not used [-Wunused-function]
 static int imx_sec_dsim_resume(struct device *dev)
            ^

So remove these dummy functions to avoid these build warnings.

Signed-off-by: Fancy Fang <chen.fang@nxp.com>