staging: drm/imx: don't drop crtc offsets when doing pageflip

CRTC offsets are only set with the initial modeset, any
subseqent pageflips assume them to be kept the same, so
we need to remember the current state until another modeset
changes it.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>