drm: rcar-du: Fix CRTC timings when CMM is used

When the CMM is enabled, an offset of 25 pixels must be subtracted from
the HDS (horizontal display start) and HDE (horizontal display end)
registers. Fix the timings calculation, and take this into account in
the mode validation.

This fixes a visible horizontal offset in the image with VGA monitors.
HDMI monitors seem to be generally more tolerant to incorrect timings,
but may be affected too.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>