drm/i915: Nuke intel_pre_disable_primary_noatomic()

Let's just inline intel_pre_disable_primary_noatomic() into
intel_plane_disable_noatomic(). The CxSR disable we can do
regardless of which plane we're disabling, and while at it we can
make the gen2 underrun w/a accurate by consulting the active_planes
bitmask.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127190556.1574-7-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>