drm/i915: clean up interlaced pipeconf bit definitions

- Clarify which bits are for which chips.
- Note that gen2 can't do interlaced directly (only via dvo tv chips).
- Move the mask to the top to make it clearer how wide this field is.
- Add defintions for all possible values.

This patch doesn't change any code.

v2: Paulo Zanoni pointed out that the pixel doubling modes do no
longer exist on ivb.

Cc: Peter Ross <pross@xvid.org>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Christopher Egert <cme3000@gmail.com>
Tested-by: Alfonso Fiore <alfonso.fiore@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>