drm/i915: refactor sink bpp clamping

As a prep work to fix it up:
- Use intel_connector instead of drm_connector to avoid too much
  upcasting in the bugfix patch.
- Extract the connector bpp clamping from the loop-over-connectors
  logic.
- Bikeshed function names (to make it clearer that
  acompute_baseline_pipe_bpp runs in the compute stage of the modeset
  sequence) and add a comment to make it clearer what it does.

No functional change in this patch.

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