drm/i915/display: Use joined pipes in intel_mode_valid_max_plane_size

drm/i915/display: Use joined pipes in intel_mode_valid_max_plane_size

In preparation of ultrajoiner, use number of pipes in the
intel_mode_valid_max_plane_size helper, instead of joiner flag.

v2: Use num_joined_pipes 1 where there are no joined pipes (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240916102836.2149012-4-ankit.k.nautiyal@intel.com