So far, pixman was used exclusively when the Chamelium support was enabled.

However, since we're going to use it as one of the backend to do the
igt_fb conversions between formats, we'll need it all the time. Make
that explicit.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>