drm/mediatek/mtk_drm_drv: Staticise local function invoked by reference

drm/mediatek/mtk_drm_drv: Staticise local function invoked by reference

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

 drivers/gpu/drm/mediatek/mtk_drm_drv.c:316:24: warning: no previous prototype for ‘mtk_drm_gem_prime_import’ [-Wmissing-prototypes]

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: YT SHEN <yt.shen@mediatek.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>