drm/i915: fold in IS_PNV checks from the split up find_dpll functions

Since I stand by my rule that splitting functions should only do an
exact copy, this is a follow-up patch.

Suggested-by: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>