drm/i915: Split PNV version of crtc_compute_clock()

Split a pnv_crtc_compute_clock(), so the differences between platforms
become more obvious.

With this, there are no more users of intel_limit() or the ->find_dpll()
hook, so get rid of them.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-16-git-send-email-ander.conselvan.de.oliveira@intel.com