drm/i915: Use crtc_state->active in primary check_plane func

Since

commit 8c7b5ccb
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Tue Apr 21 17:13:19 2015 +0300

    drm/i915: Use atomic helpers for computing changed flags

we compute the plane state for a modeset before actually committing
any changes, which means crtc->active won't be correct yet. Looking at
future work in the modeset conversion targetting 4.3 the only places
where crtc_state->active isn't accurate is when disabling other CRTCs
than the one the modeset is for (when stealing connectors). Which
isn't the case here. And that's also confirmed by an audit, we do
unconditionally update crtc_state->active for the current pipe.

We also don't need to update any other plane check functions since we
only ever add the primary state to the modeset update right now.

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>