drm/imx: ipuv3-plane: fix YUV framebuffer scanout on the base plane

Historically, only RGB framebuffers could be assigned to the primary
plane. This changed with universal plane support. Since no colorspace
conversion was set up for the IPUv3 full plane, assigning YUV frame
buffers to the primary plane caused incorrect output.
Fix this by enabling color space conversion also for the primary plane.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>