drm/i915: Remove non-existent pipes from bigjoiner pipe mask

drm/i915: Remove non-existent pipes from bigjoiner pipe mask

bigjoiner_pipes() doesn't consider that:
- RKL only has three pipes
- some pipes may be fused off

This means that intel_atomic_check_bigjoiner() won't reject
all configurations that would need a non-existent pipe.
Instead we just keep on rolling witout actually having
reserved the slave pipe we need.

It's possible that we don't outright explode anywhere due to
this since eg. for_each_intel_crtc_in_pipe_mask() will only
walk the crtcs we've registered even though the passed in
pipe_mask asks for more of them. But clearly the thing won't
do what is expected of it when the required pipes are not
present.

Fix the problem by consulting the device info pipe_mask already
in bigjoiner_pipes().

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221118185201.10469-1-ville.syrjala@linux.intel.com
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>