drm/i915: Nuke crtc->wm.cxsr_allowed

Remove crtc->wm.cxsr_allowed and just rely on crtc_state->disable_cxsr
instead. This was used only by vlv/chv to indicate whether to enable
cxsr in the wm computation. That doesn't really work anymore, and as far
as the optimal watermarks go we'll just consider the number of planes
and the current pipe, and for the intermediate watermarks we'll also
start to consider disable_cxsr which is set appropriately when planes
are being enabled/disabled.

We'll also flip over the crtc_state->wm.need_postvbl_update setup so
that it's the wm code that will set it. Previously the generic code set
it up, and then the wm code cleared it again if it thought it's not
needed after all.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-11-ville.syrjala@linux.intel.com