drm/exynos: gsc: fix spelling mistakes

Trivial fixes to spelling mistakes "precalser" to "prescaler"
in dev_err messages

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>