drm/exynos: fix missing decrement of retry counter

Currently the retry counter is not being decremented, leading to a
potential infinite spin if the scalar_reads don't change state.

Addresses-Coverity: ("Infinite loop")
Fixes: 280e54c9 ("drm/exynos: scaler: Reset hardware before starting the operation")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>