drm/i915: Make the PIPESRC rect relative to the entire bigjoiner area

drm/i915: Make the PIPESRC rect relative to the entire bigjoiner area

When using bigjoiner it's useful to know the offset of each
individual pipe in the whole set of joined pipes. Let's include
that information in our PIPESRC rectangle. With this we can make
the plane clipping code blissfully unaware of bigjoiner usage, as
all we have to do is remove the pipe's offset from the final plane
destination coordinates.

v2: Use intel_bigjoiner_num_pipes()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-14-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>