drm/i915/dsc: abstract helpers to get bigjoiner primary/secondary crtc

Add a single point of truth for figuring out the primary/secondary crtc
for bigjoiner instead of duplicating the magic pipe +/- 1 in multiple
places.

Also fix the pipe validity checks to properly take non-contiguous pipes
into account. The current checks may theoretically overflow
i915->pipe_to_crtc_mapping[pipe], albeit with a warning, due to fused
off pipes, as INTEL_NUM_PIPES() returns the actual number of pipes on
the platform, and the check is for INTEL_NUM_PIPES() == pipe + 1.

Prefer primary/secondary terminology going forward.

v2:
- Improved abstractions for pipe validity etc.

Fixes: 8a029c11 ("drm/i915/dp: Modify VDSC helpers to configure DSC for Bigjoiner slave")
Fixes: d961eb20

 ("drm/i915/bigjoiner: atomic commit changes for uncompressed joiner")
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Manasi Navare <manasi.dl.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610090528.20511-1-jani.nikula@intel.com
(cherry picked from commit 17203224

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>