drm/i915: clear up backlight #define confusion on gen4+

- Regroup definitions for BLC_PWM_CTL so that they're all together and
  and ordered according to the bitfields.

- Add all missing definitions for BLC_PWM_CTL2.

- Use the BLM_ (for backlight modulation) prefix consistently.

- Note that combination mode (i.e. also taking the legacy backlight
  control value from pci config space into account) is gen4 only.

- Move the new registers for PCH-split machines up, they're an almost
  match for the gen4 defitions.  Prefix the special PCH-only bits with
  BLM_PCH_. Also add the pipe C select bit for ivb.

- Rip out the second pair of PCH polarity definitions - they're only
  valid on early (pre-production) ilk silicon.

- Adapt the existing code to use the new definitions. This has the
  nice benefit of killing a magic (1 << 30) left behind be Jesse
  Barnes.

No functional changes in this patch.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>