drm/i915: Fixup non-24bpp support for VGA screens on Haswell

The LPT PCH only supports 8bpc, so we need to force the pipe bpp
to the right value.

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