drm/i915: Switch to common shared dpll framework for WRPLLs

Mostly this patch is one big excersize in deleting code and asserts
which are no longer needed. Note that we still abuse the shared dpll
framework a bit since we call the enable/disable functions from the
crtc mode_set and off hooks. But changing the actual hardware sequence
will be done in the next step.

Note that besides the massive amount of changes in this patch the
places and order in which the low-level WRPLL code is called is
absolutely unchanged.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[imre: rebased on patchset version w/o pch/crt/fdi refactoring]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>