drm/mediatek: don't open-code drm_gem_fb_create

Aside: There's a few other fb_create implementations which
simply check for valid buffer format (or an approximation thereof),
and then call drm_gem_fb_create. For atomic drivers at least we could
walk all planes and make sure the format/modifier combo is valid,
and remove even more code.

For non-atomic drivers that's not possible, since the format list for
the primary buffer might be garbage (and most likely it is).

Also delete mtk_drm_fb.[hc] since it would now only contain one
function.

Acked-by: CK Hu <ck.hu@mediatek.com>
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: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191115092120.4445-4-daniel.vetter@ffwll.ch