drm/i915: remove unused i915->active_pipes

Apparently the last reader of i915->active_pipes was removed with commit
ef79d62b ("drm/i915: Encapsulate dbuf state handling harder"), and
now it's only ever written to. Remove it completely.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210826141830.889-1-jani.nikula@intel.com