drm/i915: split out Ironlake pipe bpp picking code

Figuring out which pipe bpp to use is a bit painful.  It depends on both
the encoder and display configuration attached to a pipe.  For instance,
to drive a 24bpp framebuffer out to an 18bpp panel, we need to use 6bpc
on the pipe but also enable dithering.  But driving that same
framebuffer to a DisplayPort output on another pipe means using 8bpc and
no dithering.

So split out and enhance the code to handle the various cases, returning
an appropriate pipe bpp as well as whether dithering should be enabled.

Save the resulting pipe bpp in the intel_crtc struct for use by encoders
in calculating bandwidth requirements (defaults to 24bpp on pre-ILK).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>