drm/i915: Fix initial pipe underrun state tracking

Since

commit 5c673b60
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Mar 7 20:34:46 2014 +0100

    drm/i915: Don't enable display error interrupts from the start

we don't enable underrun interrupts any more at takeover time.
Unfortunately I've forgotten to also adjust the sw-side tracking.

Since the code assumes that disabled pipes have underrun reporting
enabled set the disable flag only on all pipes which are active at
takeover time. Without this underrun reporting wasn't enabled
correctly on the first modeset. Note that for fastboot this is another
piece of state that needs to be fixed up by enabling the underrung
reporting after watermarks have beend fixed up.

On ivb/hsw an additional effect of this regression was that also all
cpu crc reporting stopped working since the master error interrupt it
shared across all pipes and sources.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76150


[danvet: Augment the code comment and polish the commit message a bit,
as discussed with Jani.]
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: lu hua <huax.lu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>