media: v4l2-common: change the pixel_enc of V4L2_PIX_FMT_GREY to YUV

V4L2_PIX_FMT_GREY format is Ycbcr format without
the color data, therefore its pixel_enc should
set to V4L2_PIXEL_ENC_YUV.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>