drm/i915: Fold the DPLL limit defines into the structs that use them.

They're used in one place, and not providing any descriptive value,
with their names just being approximately the conjunction of the
struct name and the struct field.

This diff was produced with gcc -E, copying the new struct definitions
out, moving a couple of the old comments into place in the new
structs, and reindenting.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>