drm/i915: pipe_src_w must be even in LVDS dual channel, DVO ganged, and double wide mode

Pipe horizontal source size must be even when either LVDS dual channel
mode, DVO ganged mode, or pipe double wide mode is used.

We must round it down since we can never increase the user specified
viewport size.

The actual error from an odd pipe source width looks like a diagonal
shift, like you might get from a bad stride.

v2: s/ganaged/ganged/

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>