drm/i915: Use crtc_state->active instead of crtc_state->enable

crtc_state->enable means a crtc is configured, but it may be turned
off for dpms. Until the commit "use intel_crtc_control everywhere"
crtc_state->active was not updated on crtc off, but now
crtc_state->active should be used for tracking whether a crtc is
scanning out or not.

A few commits from now dpms will be handled by calling
intel_set_mode with a different value for crtc_state->active,
which causes a crtc to turn on or off.

At this point crtc->active should mirror crtc_state->active,
so some paranoia from the crtc_disable functions can be removed.

intel_set_mode_setup_plls still checks for ->enable, because all
resources that are needed have to be calculated, else
dpms changes may not succeed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>