drm/i915: Simplify the way BC bifurcation state consistency is kept

Remove the global modeset resource function that would disable the
bifurcation bit, and instead enable/disable it when enabling the pch
transcoder. The mode set consistency check should prevent us from
disabling the bit if pipe C is enabled so the change should be safe.

Note that this doens't affect the logic that prevents the bit being
set while a pipe is active, since the patch retains the behavior of
only chaging the bit if necessary. Because of the checks during mode
set, the first change would necessarily happen with both pipes B and
C disabled, and any subsequent write would be skipped.

v2: Only change the bit during pch trancoder enable. (Ville)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>