drm/i915: convert device info num_pipes to pipe_mask

Replace device info number of pipes with a bit mask of available
pipes. This will prove handy in the future. There's still a bunch of
future work to do to actually allow a non-consecutive mask of pipes, but
it's a start. No functional changes.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911202908.19631-1-jani.nikula@intel.com