drm/vc4: plane: Expand the lower bits by repeating the higher bits

The vc4 HVS uses an internal RGB888 representation of the frames, and will
by default expand formats using a lower depth using zeros.

This causes an issue when we try to use other compositing software such as
pixman that fill the missing bits by repeating the higher significant bits.
As such, we can't check the display output in a reliable way by doing a
software composition and an hardware one and compare both.

To prevent this, force the same behaviour so that we can do such things.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517133759.25626-1-maxime.ripard@bootlin.com