drm/i915: print computed bpp in dp link configuration

Pretty useful to debug our DP bandwidth woes.

v2: Also print out the required and available link bandwidth,
suggested by Chris Wilson.

v3: Also print out the input parameters so that diagnosing failures to
find a valid dp link configuration is possible.

v4: s/Display port/DP/ to shorten the output.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>