drm/mediatek: select DRM_GEM_DMA_HELPER if DRM_FBDEV_EMULATION=y

With the recent switch from fbdev-generic to fbdev-dma, the driver now
requires the DRM GEM DMA helpers. This dependency is missing, and will
cause a link failure if fbdev emulation is enabled.

Add the missing dependency.

Fixes: 0992284b ("drm/mediatek: Use fbdev-dma")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Chun-Kuang Hu <chunkuang.hu@mediatek.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240620054708.2230665-1-wenst@chromium.org