drm/i915: Only dither on 6bpc panels

In

commit d328c9d7
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Apr 10 16:22:37 2015 +0200

    drm/i915: Select starting pipe bpp irrespective or the primary plane

we started to select the pipe bpp from sink capabilities and not from
the primary framebuffer - that one might change (and we don't want to
incur a modeset) and sprites might contain higher bpp content too.

We also selected dithering on a 8 bpc screen displaying a 24bpp rgb
primary, because pipe_bpp is 24 for such a typical 8 bpc sink, but since
the commit mentioned above, base_bpp is always the absolute maximum
supported by the hardware, e.g., 36 bpp on my Ironlake chip. Iow. the
only way to not get dithering would have been to connect a deep color 12
bpc display, so pipe_bpp == 36 == base_bpp.

Hence only enable dithering on 6bpc screens where we difinitely and
always want it.

Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Reported-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>