media: tpg: change char argument to const char

Change the argument of type char * to const char * for function
tpg_gen_text().

This function should take in a const char * as opposed to char * as it
does not make changes to the text. This issue was found while passing
the order of colors of tpg generated test image (which is a const char
*) to this function.

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