drm/i915: switch crtc->shared_dpll from a pointer to an enum

Dealing with discrete enum values is simpler for hw state readout and
pipe config computations than pointers - having neat names instead of
chasing pointers should look better in the code.

This isn't a that good reason for pch plls, but on haswell we actually
have 3 different types of plls: WRPLL, SPLL and the DP clocks. Having
explicit names should help there.

Since this also adds the intel_crtc_to_shared_dpll helper to further
abstract away the crtc -> dpll relationship this will also help to
make the next patch simpler, which moves the shared dpll into the pipe
configuration.

Also note that for uniformity we have two special dpll ids: NONE for
pipes which need a shared pll but don't have one (yet) and private for
when there's a non-shared pll (e.g. per-pipe or per-port pll).

I've thought whether we should also add a 2nd enum for the type of the
pll we want (for really generic pll selection code) but thrown that
idea out again - likely there's too much platform craziness going on
to be able to share the pll selection logic much.

Since this touched all the shared_pll functions a bit I've also done
an s/intel_crtc/crtc/ replacement on a few of them.

v2: Kill DPLL_ID_NONE. It's probably better to call it DPLL_ID_INVALID and use
it to check that the compute config stage assigns a dpll to every pipe.
But since that code isn't ready yet until we move the dpll selection out
of the ->mode_set callback, there's no use for it.

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