drm/i915: Introduce intel_crtc_joined_pipe_mask()

drm/i915: Introduce intel_crtc_joined_pipe_mask()

Add a small helper to compute the set of pipes that the current
crtc is using.

And we have at least one trivial place in
intel_ddi_update_active_dpll() where we can use it
immediately, so let's do that.

v2: Use the name 'pipe_crtc' for the per-pipe crtc pointer

Tested-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404213441.17637-12-ville.syrjala@linux.intel.com