drm/exynos: fix build warning.

This patch fixes erroneous register read.
reall function needs register base address + offset but
exynos_drm_gsc module used only offset to read a register.
so this patch uses gsc_read function instead of readl.
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_gsc.c

@@ -1592,7 +1592,7 @@ static int gsc_ippdrv_start(struct device *dev, enum drm_exynos_ipp_cmd cmd)
 		exynos_drm_ippnb_send_event(IPP_SET_WRITEBACK, (void *)&set_wb);
 
 		/* src local path */
-		cfg = readl(GSC_IN_CON);
+		cfg = gsc_read(GSC_IN_CON);
 		cfg &= ~(GSC_IN_PATH_MASK | GSC_IN_LOCAL_SEL_MASK);
 		cfg |= (GSC_IN_PATH_LOCAL | GSC_IN_LOCAL_FIMD_WB);
 		gsc_write(cfg, GSC_IN_CON);