drm/i915: Clean up the gen2 "no planes -> underrun" workaround

We have the active_planes bitmask now so use it to properly
determine when some planes are visible for the gen2 underrun
workaround.

This let's us almost eliminate intel_post_enable_primary().
The manual underrun checks we can simply move into
intel_atomic_commit_tail() since they loop over all the pipes
already. No point in repeating the checks multiple times when
there are multiple pipes in the commit.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127190556.1574-6-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>