staging: drm/imx: handle framebuffer offsets correctly

We didn't take the pixel format into account, so x-direction
offsets were off by a factor of 2 or 4 for 16bpp and 32bpp
framebuffers.

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>