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>