drm/i915: refactor PCH_DPLL_SEL #defines

The bits are evenly space, so we can cut down on two big switch
blocks. This also greatly simplifies the hw state readout which
follows in the next patch.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>