drm/exynos/exynos7_drm_decon: Fix incorrect naming of 'decon_shadow_protect_win()'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/exynos/exynos7_drm_decon.c:355: warning: expecting prototype for shadow_protect_win(). Prototype was for decon_shadow_protect_win() instead

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc: Akshu Agarwal <akshua@gmail.com>
Cc: Ajay Kumar <ajaykumar.rs@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602143300.2330146-24-lee.jones@linaro.org