The chamelium format subtests will need to convert the reference pattern to
the format to be tested on the DRM device.

However, Cairo is very limited when it comes to format, and while pixman
has much more support for formats, it's still falling short compared to
what DRM exposes, especially on the YUV side. Plus, since we want to run
CRC checks on the frame, we can't afford having conversions back and forth
between RGB24, as the current API is doing.

In order to abstract this away, let's create a function that will convert a
igt_fb structure to another DRM format and return the converted igt_fb.

For now, we will use only cairo to do the conversion, but we will use other
libraries or roll our own routines to convert to more exotic formats and
abstract it away from the users.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>