drm/i915: Always keep crtc_state->active in sync with enable

With the recent modeset internal rework, we wind up setting crtc_state->enable
to false, but leave crtc_state->active as true following a
drmModeSetCrtc(fb=0), which is incorrect.  This mismatch gets caught by
drm_atomic_crtc_check() and causes subsequent atomic operations (such as plane
updates while the CRTC is disabled) to fail.

Bisect points to

        commit dad9a7d6d96630182fb52aae7c3856e9e7285e13
        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

as the commit that actually triggers the regression.

The difference compared to (which this patch reverts)

commit 90d46906


Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Thu May 7 14:31:28 2015 -0700

    drm/i915: Set crtc_state->active to false when CRTC is disabled (v2)

is that we know keep state->active/enable in sync for all legacy
modeset paths, as it should be.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reported-and-Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Directly squash in the revert and augment the commit
message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Revert "drm/i915: Set crtc_state->active to false when CRTC is disabled (v2)"

This reverts commit 90d46906.