drm/i915: don't call Haswell PCH code when we can't or don't need

On Ironlake we have one PCH transcoder and FDI per pipe, so we know
that if ironlake_crtc_driving_pch returns false we can disable the PCH
transcoder and we also know that when we disable the crtc we can also
disable the PCH transcoder.

On Haswell there is only 1 PCH transcoder and FDI and they can be used
by any CRTC. So if for one specific crtc haswell_crtc_driving_pch
returns false we can't assert anything about the state of the PCH
transcoder or the FDI link without checking if any other CRTC is using
the PCH.

So on this commit remove the "assert_fdi_{t,r}x_disabled" form
haswell_crtc_enable and also only disable FDI and the PCH transcoder
if the port being disabled was actually a PCH port (we only have one
port using PCH: the VGA port).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>