media: v4l2-tpg: add HDMI Video Guard Band test pattern

This inserts 4 pixels of the RGB color 0xab55ab at the left hand side of
the image. This is only done for 3 or 4 byte RGB pixel formats. The HDMI
TMDS encoding of this pixel value equals the Video Guard Band value as
defined by HDMI (see section 5.2.2.1 in the HDMI 1.3 Specification) that
preceeds the first actual pixel of a video line. If an HDMI receiver
doesn't handle this correctly, then it might keep skipping these Video
Guard Band patterns and end up with a shorter video line. So this is a
nice pattern to test with.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>