drm/mediatek: Fix build break

Caused by file removal without adjusting the Makefile.

Fixes: d268f42e ("drm/mediatek: don't open-code drm_gem_fb_create")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127170513.42251-1-mihail.atanassov@arm.com