drm/i915: Move SPLL disabling into hsw_crt_post_disable

Similar to how the ->crtc_mode_set hook should touch the hardware to
enable anything the ->crtc_off hook should disable anything in the
hardware. Otherwise runtime pm for dpms will not work.

Currently the only things left int the haswell_crtc_off hook is
disabling the ddi plls. We can't move the WRPLL enabling out yet
because the current ddi pll sharing code used by the haswell code
doesn't separately track active users and overall users. This must be
fixed by porting it to the generic shared display pll framework, which
is powerful enough.

But the SPLL source is only used by the crt encoder and so can be
moved already. We only need to make sure that the ddi port E is
already off, which hsw_fdi_disable does by calling
intel_ddi_post_disable.

With this the code reorg to shuffle hsw fdi/lpt specific code into a
new hsw-specific crt encoder type is now finally complete.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[imre: rebased on patchset version w/o pch/crt/fdi refactoring]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>