Currently there is no method to know the correct order of the colors for
a test image generated by tpg. Write a function that returns a string of
colors' order given a tpg. It returns a NULL pointer in case of test
patterns which do not have a well defined colors' order. Hence add a
NULL check for text in tpg_gen_text().

[hverkuil: white -> White (for consistency)]
Signed-off-by: Kaaira Gupta <kgupta@es.iitr.ac.in>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>