drm/i915: Replace ILK eDP underrun suppression with something better

The underruns we were seeing when enabling eDP port A on ILK seem to
have been caused by prematurely clearing the LP1+ watermark values when
disabling LP1+ watermarks. Now that the watermarks are handled
properly, we can rip out the underrun suppression around the port A
enable.

We still need to worry about the underruns on FDI when enabling
the eDP PLL. But as Bspec tells us, we can avoid that by a vblank
wait on the pipe driving FDI just prior to enabling the eDP PLL.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459536799-18109-4-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>