drm/i915: split out intel_pnv_find_best_PLL

Pineview is just different.

Also split out i9xx_clock from intel_clock and drop the now redundant
struct device * parameter.

Note that in this patch I kill an XXX comment about 100MHz clocks. I
couldn't figure out what this is about, and we don't seem to have any
bug reports about this either. I suspect that it's a remnant from when
the i9xx and ilk+ modeset code was all in the same file since ilk+
does indeed have a 100MHz clock. So I've just killed it to stop the
cargo-culting.

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