drm/exynos: correct exynos_drm_fimd kerneldoc

Correct the kerneldoc of fimd_shadow_protect_win() to fix W=1 warnings:

  drivers/gpu/drm/exynos/exynos_drm_fimd.c:734: warning:
    expecting prototype for shadow_protect_win(). Prototype was for fimd_shadow_protect_win() instead

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>