drm/i915/dp: Allow big joiner modes in intel_dp_mode_valid(), v3.

Small changes to intel_dp_mode_valid(), allow listing modes that
can only be supported in the bigjoiner configuration, which is
not supported yet.

v13:
* Allow bigjoiner if hdisplay >5120
v12:
* slice_count logic simplify (Ville)
* Fix unnecessary changes in downstream_mode_valid (Ville)
v11:
* Make intel_dp_can_bigjoiner non static
so it can be used in intel_display (Manasi)
v10:
* Simplify logic (Ville)
* Allow bigjoiner on edp (Ville)
v9:
* Restric Bigjoiner on PORT A (Ville)
v8:
* use source dotclock for max dotclock (Manasi)
v7:
* Add can_bigjoiner() helper (Ville)
* Pass bigjoiner to plane_size validation (Ville)
v6:
* Rebase after dp_downstream mode valid changes (Manasi)
v5:
* Increase max plane width to support 8K with bigjoiner (Maarten)
v4:
* Rebase (Manasi)

Changes since v1:
- Disallow bigjoiner on eDP.
Changes since v2:
- Rename intel_dp_downstream_max_dotclock to intel_dp_max_dotclock,
  and split off the downstream and source checking to its own function.
  (Ville)
v3:
* Rebase (Manasi)

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[vsyrjala:
* Keep bigjoiner disabled until everything is ready
* Appease checkpatch]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-3-manasi.d.navare@intel.com