[media] media: mtk-mdp: support pixelformat V4L2_PIX_FMT_MT21C

Add V4L2_PIX_FMT_MT21C in format list.

[mchehab@s-opensource.org: re-add mtk_mdp_size_align]

Signed-off-by: Minghsiu Tsai <minghsiu.tsai@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>